Fix GDScriptAnalyzer::is_shadowing() missing warnings for shadowed global constants

[EdwardChanCH]

Closes: #117567

Related: #106987

Note: Some previous code has been split into #117545 to keep this PR manageable.

Any help is appreciated!

[EdwardChanCH]

Converted to Draft PR, still discovering additional bugs to be fixed:

• Local variables/ constants can shadow global enum classes or members
• Local unnamed enum members can shadow global enum classes or members
• Local named enum members have incorrect tooltips (and colours) if it is named PI, TAU, INF, or NAN
• Local named enum classes can shadow global enum members when printed (e.g. enum OK { }, print(OK))
• Local named enum classes can shadow global enum classes (e.g. Error.OK)
• Local functions have incorrect tooltips (and colours) if it is named PI, TAU, INF, or NAN
• Local functions can shadow global constants when printed (e.g. enum PI { }, print(PI))

Edit: Fixed.

[EdwardChanCH]

Fixed all of the bugs listed above. Will add more test cases next.

Notes:

• The [Ignore] warning button inserts @warning_ignore("shadowed_global_identifier") above/inside enum declarations, which produces parse compile errors.
  • A workaround is to surround the enum declaration with @warning_ignore_start("shadowed_global_identifier") + @warning_ignore_restore("shadowed_global_identifier").

The only notable side effect of the new enum warnings, unsure if it needs to be implemented in this PR.

Edit: This will be fixed in a future PR.

[EdwardChanCH]

Added more test cases and completed manual testing. Ready for review.

[EdwardChanCH]

The unit tests all passed, but some leaked memory.

Edit: Fixed.

[EdwardChanCH]

I will refactor this PR soon, so this will be made a draft until then.

Edit: This PR now focuses on adding warnings for shadowed global constants.

Less relevant code has been moved to other PRs:
#117545 (fix tooltip for shadowed global constants)
#117308 (fix tooltip for shadowed global functions)
#106987 (add warnings for shadowed global functions)

[EdwardChanCH]

Below is a full summary of what this PR fixed, and what will be fixed in future issues/proposals:
godotengine/godot-proposals#14425

class_name _Main
extends Node

var PI = -10 # Missing warning. <-- Fixed! Now warns (SHADOWED_GLOBAL_IDENTIFIER)
var OK = -20 # Missing warning. <-- Fixed! Now warns (SHADOWED_GLOBAL_IDENTIFIER)
enum Error { # Missing warning. --> Proposal #14425
	OK = -30 # Missing warning. --> Proposal #14425
}
var Side = -40 # Missing warning. <-- Fixed! Now warns (SHADOWED_GLOBAL_IDENTIFIER)
var MouseButtonMask # Missing warning. <-- Fixed! Now warns (SHADOWED_GLOBAL_IDENTIFIER)
var number = -60 # No warning.

[EdwardChanCH]

Accidentally closed the PR...