Add no-restricted-syntax rule for union type assertions (no as)

[omerlh]

Adds a no-restricted-syntax rule that disallows TypeScript union type assertions (value as A | B). The rule suggests using type guards, narrowing, or typed constructors instead. Non-union assertions (e.g. as const) remain allowed.

Also fixes the pre-existing lint failure in fixtures/ts.ts where @typescript-eslint/unified-signatures threw on that file; the rule is disabled for that fixture in the sample configs.

Review: @noam — could you review when you have a moment?

Made with Cursor

[noamokman]

Why turn this off?

[omerlh]

@noamokman Re your question on turning off unified-signatures: we had to disable it for fixtures/ts.ts because the rule was throwing (typeParameters.params is not iterable) when linting that file—a bug in the rule with that fixture’s AST. After upgrading to the latest deps, the crash is gone, so I removed that workaround in the latest commit. The rule now runs on the fixture without issues.