Add anchor sliding along adjacent segments in the Path tool

[4adex]

Partly closes #2715

This is currently implemented with CtrlShift drag, but will move to GG in a followup PR.

[Keavon]

• Please add Esc/RMB cancellation.
• And the hint, which can be "+ CtrlShift Slide" added to "[RMB] Drag Selected" that should show up only when there is zero or one anchor, and zero handles, currently selected.

[otdavies]

Caught an error when working with this new feature (very cool btw! I think it should be double tap G like blender though)

panicked at editor/src/messages/tool/tool_messages/path_tool.rs:2266:117:
called `Option::unwrap()` on a `None` value

Error
    at https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:943:1179
    at g (https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:929:64934)
    at Object.handleJsMessage (https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:929:65082)
    at https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:929:65475
    at https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:929:12694
    at vt (https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:928:6130)
    at o.wbg.__wbg_call_833bed5770ea2041 (https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:929:12664)
    at graphite_wasm.wasm._ZN13graphite_wasm10editor_api12EditorHandle27send_frontend_message_to_js17h0668831c386ddb67E (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[3717]:0x8b8a38)
    at graphite_wasm.wasm._ZN4core3ops8function2Fn4call17h7e50954c4596dc1dE (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[14276]:0xbb570d)
    at graphite_wasm.wasm._ZN3std9panicking20rust_panic_with_hook17h1f8226e239ffaa71E (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[7593]:0xa9f925)
    at graphite_wasm.wasm._ZN4core9panicking9panic_fmt17hb6c28595a1feee80E (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[11808]:0xb8b6cd)
    at graphite_wasm.wasm._ZN4core9panicking5panic17h6d53c7d07dcdab1eE (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[11308]:0xb83404)
    at graphite_wasm.wasm._ZN4core6option13unwrap_failed17h1b26bbcca9c2733dE (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[13818]:0xba3df8)
    at graphite_wasm.wasm._ZN285_$LT$graphite_editor..messages..tool..tool_messages..path_tool..PathTool$u20$as$u20$graphite_editor..utility_traits..MessageHandler$LT$graphite_editor..messages..tool..tool_message..ToolMessage$C$$RF$mut$u20$graphite_editor..messages..tool..utility_types..ToolActionHandlerData$GT$$GT$15process_message17h57cc7359c7418186E (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[1382]:0x5d0821)
    at graphite_wasm.wasm._ZN280_$LT$graphite_editor..messages..tool..tool_message_handler..ToolMessageHandler$u20$as$u20$graphite_editor..utility_traits..MessageHandler$LT$graphite_editor..messages..tool..tool_message..ToolMessage$C$graphite_editor..messages..tool..tool_message_handler..ToolMessageData$GT$$GT$15process_message17hb9f008b410d7f07aE (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[428]:0x246a39)
    at graphite_wasm.wasm._ZN15graphite_editor10dispatcher10Dispatcher14handle_message17hef80e4234270813cE (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[440]:0x25a86f)
    at graphite_wasm.wasm._ZN13graphite_wasm10editor_api12EditorHandle8dispatch17hfd01dc31199e82deE (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[4376]:0x94a297)
    at graphite_wasm.wasm.editorhandle_onMouseUp (https://5f6ef0f3.graphite.pages.dev/assets/graphite_wasm_bg-Cw88cXvm.wasm:wasm-function[5015]:0x99ae9a)
    at G2.onMouseUp (https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:929:5974)
    at $ (https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:932:3674)
    at action (https://5f6ef0f3.graphite.pages.dev/assets/index-DVDznk7q.js:932:809)

[Keavon]

Ok, yeah, one more request please: instead of Ctrl+Shift, let's make this double-G. So when you're in GRS grabbing mode, just as you can hit keys like X, Y, etc. it should also accept G which will discard any existing translation that was applied and reset to the translation when G was first hit (in case the user moves the mouse between the two G presses), then it should enter sliding mode until LMB/Enter (confirm) or RMB/Esc (cancel) which exits GRS mode.

[Keavon]

I'm actually going to merge this now and that, plus the crash fix, can be a followup PR.