Add ergonomics helpers on Node mixin

[ggueret]

Summary

• Adds four helpers on _NodeTypeMixin (inherited by both Node and NodeProxy): children_of_type, field_text, field_of_type, is_operator.
• Centralises repetitive idioms that recur across the built-in rules: direct-child filtering by type, field-text shortcut, type-guarded field access, and operator matching across the tree-sitter type/text duality.

Test plan

• children_of_type returns only direct named children (distinct from recursive find())
• children_of_type accepts a string, NodeType enum, and an iterable of types
• field_text returns the field child's text, or None when absent
• field_of_type returns the field child only when its type matches
• is_operator matches against both node type and node text

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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