Types: Add discriminated union support across language bindings

[marcoroth]

This pull request updates the type system to support discriminated unions for AST node fields instead of using the generic Node type as the common ancestor.

The config.yml schema definitions have been updated to specify exact union types for fields like:

• HTMLElementNode.open_tag (now HTMLOpenTagNode | HTMLConditionalOpenTagNode) (see Parser: Introduce HTMLConditionalOpenTagNode #1153)
• ERBIfNode.subsequent (now ERBIfNode | ERBElseNode)
• HTMLAttributeValueNode.children (now LiteralNode | ERBContentNode)
• HTMLCloseTagNode.children (now WhitespaceNode)

The code generation templates have been updated across all language bindings. TypeScript now generates proper union types and the is*Node type guard functions have been updated to accept null | undefined for easier chaining. Ruby generates union type signatures in RBS files. Rust adds a new union_types.rs file with enum types and conversion functions for each union.

With these type improvements the linter, formatter, and language server code has been simplified to use is*Node type guards instead of manual .type === "AST_*" string checks. This removes numerous as any and as *Node casts that were previously needed, making the code more type-safe and readable.

[pkg-pr-new]

npx https://pkg.pr.new/@herb-tools/formatter@1154

npx https://pkg.pr.new/@herb-tools/language-server@1154

npx https://pkg.pr.new/@herb-tools/linter@1154

commit: c635318

[github-actions]

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