Fix class constant highlighting

[alfredomtx]

Summary

• Add explicit class_constant_access_expression rule to correctly capture the constant name (e.g., RESOURCE_RELATIONS in Class::CONSTANT) as @constant
• Reorder @constructor before @constant so UPPER_SNAKE_CASE names get @constant instead of being overridden by the generic @constructor catch-all

Problem

Class constants accessed via :: (e.g., Conversation::RESOURCE_RELATIONS) were highlighted as @constructor (same as class names) instead of @constant. This happened because:

1. The generic ((name) @constructor (#match? @constructor "^[A-Z]")) pattern matched both Conversation and RESOURCE_RELATIONS
2. Since it appeared after the @constant pattern, it overrode the constant highlighting

Fix

1. New rule: (class_constant_access_expression (_) (name) @constant) — explicitly captures the constant name within :: access expressions. Uses positional matching since this AST node has no named fields.
2. Reorder: Move @constructor before @constant so that for tokens matching both patterns, @constant (being last) wins.

Verified with tree-sitter CLI

Conversation     → @constructor ✓
RESOURCE_RELATIONS → @constant ✓
MAX_VALUE        → @constant ✓
PHP_INT_MAX      → @constant ✓
self             → @constructor ✓

Screenshots:

[alfredomtx]

Fixes item 1 of issue #81