feat: use VarKind for variable semantics

[aofei]

This distinguishes receivers, parameters, named results, and local variables in semantic tokens and spx definitions while keeping field checks on IsField() where that API is clearer. Add regression tests for TypeInfo, semantic tokens, hover overviews, and spx definitions.

Updates goplus/gogen#594
Updates goplus/xgo#2665

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.78%. Comparing base (064aca3) to head (89d67e5).

Files with missing lines	Patch %	Lines
internal/server/semantic_token.go	83.33%	2 Missing ⚠️
internal/server/var_kind.go	91.66%	1 Missing and 1 partial ⚠️
internal/server/spx_definition.go	87.50%	1 Missing ⚠️
internal/server/spx_util.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
+ Coverage   68.59%   68.78%   +0.18%     
==========================================
  Files          55       56       +1     
  Lines        7041     7066      +25     
==========================================
+ Hits         4830     4860      +30     
+ Misses       1823     1817       -6     
- Partials      388      389       +1     

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