refactor: labeled expressions implementation

[timofei-iatsenko]

This is my attempt on fixing #241 and #239

This is more systematic refactoring rather then ad-hoc fix. The refactoring make closer SWC implementation to a babel's one, so they not diverge.

Previously indexes for expression were allocated during processing a Message Phase in a message builder, in babel version those indexes where assigned during AST scanning phase, so it produced a diffrent result described in #241

Additionally ph macro was not processed uniformly. It was processed in some places but were skipped in other, for example:

<Plural value={ph({count: getCount()})} / > 

With this refactoring, ph support is added everywhere and processed uniformly.

Other changes:

1. I extracted all tests related to labeled expressions feature into separate file. It's easier to see all cases and work on the feature this way
2. I changed internal structures,

• TransformCtx (formerly MacroCtx) — global per-file context holding macro imports, options, directives, and runtime identifiers.
• MacroCtx<'a> — short-lived, created fresh for each macro invocation. Borrows &'a mut TransformCtx and owns its own expression index counter starting at 0.
• Free tokenization functions: tokenize_tpl, tokenize_expr_to_arg, try_tokenize_call_expr_as_choice_cmp, try_tokenize_expr, and get_choice_cases_from_obj are now free functions taking &mut MacroCtx explicitly, making the data flow clear.

[codecov]

Codecov Report

❌ Patch coverage is 90.83095% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.81%. Comparing base (4a68dd1) to head (7ff152f).

Files with missing lines	Patch %	Lines
crates/lingui_macro/src/macro_utils.rs	90.82%	20 Missing ⚠️
crates/lingui_macro/src/jsx_visitor.rs	83.33%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
- Coverage   95.06%   94.81%   -0.26%     
==========================================
  Files           9        9              
  Lines        2251     2334      +83     
==========================================
+ Hits         2140     2213      +73     
- Misses        111      121      +10     

Flag	Coverage Δ
unittests	94.81% <90.83%> (-0.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/lingui_macro/src/ast_utils.rs	90.80% <ø> (-0.41%)	⬇️
crates/lingui_macro/src/builder.rs	99.15% <100.00%> (+0.17%)	⬆️
crates/lingui_macro/src/js_macro_folder.rs	98.42% <100.00%> (+0.06%)	⬆️
crates/lingui_macro/src/lib.rs	91.09% <100.00%> (+0.01%)	⬆️
crates/lingui_macro/src/jsx_visitor.rs	86.63% <83.33%> (-1.81%)	⬇️
crates/lingui_macro/src/macro_utils.rs	91.90% <90.82%> (+2.69%)	⬆️

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