feat: strict, quote-aware event argument parsing with literal support

[AKnassa]

What this does

Event handler arguments in declarative templates can now be real literal values — strings, numbers, true, false, null — and anything malformed is rejected loudly instead of being quietly misread.

Why

Previously every argument that was not $e or $c was assumed to be a binding path. So handler("hello, world") was silently mangled, and a typo produced confusing behaviour at runtime instead of a clear error at parse time.

What changed

• String, number, boolean and null literals are parsed as literals
• Commas and parentheses inside quoted strings are respected
• String escape sequences (\n, \t, \\, \, and friends) are decoded; unknown ones throw
• Malformed handler syntax now throws while the template is parsed
• Docs updated

How to see it

Run the fast-element Playwright suite. 109 tests cover the parser.

Closes #7567

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