Improve global variable alias error messaging

[allcre]

Improve error message for invalid global variable aliasing

Summary

This PR improves the error message when attempting to alias a global variable with a non-global variable. The previous generic error message was misleading and has been replaced with a more specific and descriptive one.

The Problem

The old error message was confusing because it didn't explain the actual constraint. Consider this example:

alias $a b
         ^ invalid argument being passed to `alias`; expected a bare word, symbol, constant, or global variable

This error message is misleading because:

1. b is a bare word, which the error says is expected
2. The message doesn't explain the actual rule: when aliasing global variables, both names must be global variables

The Solution

The new error message clearly states the actual requirement:

alias $a b
         ^ invalid argument being passed to `alias`; when aliasing global variables, both the old and new names must be global variables

Changes

• Added new error constant PM_ERR_ALIAS_ARGUMENT_GLOBAL_VARIABLE to provide clearer error messaging
• Updated the parser to use the new error when aliasing global variables with invalid arguments
• Updated test case aliasing_global_variable_with_non_global_variable.txt to verify the new error message

[st0012]

I think the message can be more specific. Since this error only happens when the first (new) name is already a global variable, we can probably say

invalid argument being passed to `alias`; expected the old name to be a global variable as well

[eregon]

old name might be unclear if it's the first or second argument (alias is notoriously confusing about this), so I'd suggest to use the second name instead of the old name, or the message in the current state of this PR (which sounds clear to me).

[st0012]

That's fair. My point is mostly that the message can be more concise. the second name indeed works better.