Expression parsers: accept UAX 31 identifiers in paths, function names and argument names

[gspencergoog]

The v1.0 specification requires catalog component, function and argument names to match ^[\p{XID_Start}_][\p{XID_Continue}]*$. See "Catalog Entity Naming Rules" in specification/v1_0/docs/a2ui_protocol.md. The expression parsers do not follow that rule yet:

• TypeScript, Python and Dart accept only ASCII in paths (a-z, A-Z, 0-9, /, ., _, -) and identifiers. ${café} fails with Unexpected characters, and an expression cannot call a catalog function named größe.
• Swift uses Character.isLetter and Character.isNumber. It accepts ${café}, but it also accepts characters outside UAX #31, such as ½.

Proposed change (follow-up to #2807):

• Function and argument names follow UAX #31: XID_Start or _, then XID_Continue.
• Path segments allow UAX #31 identifier characters and keep /, ., _ and -.
• After true, false and null, any XID_Continue character counts as part of the word.
• TypeScript and Dart scan by Unicode code point instead of UTF-16 code unit.
• Reuse typescript/web_core/src/common/uax31.ts and python/a2ui_core/src/a2ui/core/common/uax31.py. Add UAX #31 helpers for Dart and for Swift (Unicode.Scalar.Properties.isXIDStart and isXIDContinue).
• Add conformance cases in conformance/core/expressions.yaml for a non-ASCII path, function name, argument name, and rejected non-UAX #31 character.
• Optionally add a specification paragraph that defines the expression grammar, including number literals.

Related: no parser accepts @ in a function name, so ${@index()} fails. Decide whether expressions can call reserved @ functions.