Collect sections in calls and emit them as document symbols

[lionel-]

Branched from #866.
Addresses posit-dev/positron#8402.

You can now add sections in function calls too, between arguments.

The implementation is shared with the section logic for {} blocks. This works a little differently than in RStudio: The sections are nested in the call and can't affect the section hierarchy outside the call. If you have:

## level 2 ----

call(
  # level 1 ----
)

call({
  # level 1 ----
})

The "level 1" sections in the block and call do not close the top-level "level 2" section. This difference is necessary because the LSP outline is more powerful and includes syntactic elements like function definitions and assigned variables in the outline. Allowing sections to close from inside a nested element would make things complicated and I'd argue respecting the nesting of the code makes sense from a user pespective too.

QA Notes

Sections in calls:

# level 1 ----

list(
  ## foo ----
  1,
  2, ## bar ----
  3,
  4
  ## baz ----
)

## level 2 ----

list(
  # foo ----
  1,
  2, # bar ----
  3,
  4
  # baz ----
)

now work the same way as sections in blocks:

# level 1 ----

list({
  ## foo ----
  1
  2 ## bar ----
  3
  4
  ## baz ----
})

## level 2 ----

list({
  # foo ----
  1
  2 # bar ----
  3
  4
  # baz ----
})

Screen.Recording.2025-07-07.at.15.31.07.mov