GDScript: Add warning for shadowing global constants

[acs110113]

Description

This PR adds a missing warning in GDScript when a local variable or property shadows a global constant (such as PI, OK, or INF).

Previously, shadowing these global constants would happen silently, which could lead to confusing logic errors for users who expect to be using the built-in global values.

Fixes

Changes

• In gdscript_analyzer.cpp, added a check against GDScriptLanguage::get_singleton()->get_global_map() within the is_shadowing validation logic.
• Trigger a SHADOWED_VARIABLE warning if a match is found in the global scope.

Validation

Tested with the following script:

var PI = 3.14 # Now triggers a warning
var OK = 0    # Now triggers a warning

[dalexeev]

• See Fix GDScriptAnalyzer::is_shadowing() missing warnings for shadowed global constants #106984.

[acs110113]

"Thanks for pointing that out! I'm glad to see this is being addressed in #106984. As this was my first attempt at contributing to Godot, I've learned a lot through the process. I'll close this PR in favor of the existing fix."