Implement erasable enum annotations - #65421

The goal is to provide stronger type constraints than an ordinary object without requiring an enum-specific runtime transform. As described in #60790, the as const object pattern requires a verbose typeof X[keyof typeof X] alias and produces a plain union rather than a nominal enum type.

This PR adds erasable : enum annotations, combining ordinary JavaScript object literals with enum types and named member types:

export const Mode: enum = {
    Read: "read",
    Write: "write",
};

let mode: Mode = Mode.Read;
let read: Mode.Read = Mode.Read;

The implementation separates parsing, type checking, and emission:

• Parsing and AST: Reuse the existing type-reference and object-literal syntax. No parser implementation changes or new object AST nodes are required. The binder and checker recognize the : enum annotation.
• Type checking: Give the declaration an associated nominal enum type and readonly member types. Member shape and value restrictions are enforced by the checker to keep TypeScript’s type model consistent.
• JavaScript emission: Erase the annotation while preserving ordinary object initialization, expressions, and member accesses. Downstream transpilers and type strippers do not need to reproduce TypeScript’s semantic checks or perform an enum-specific runtime transformation. The syntax satisfies the erasableSyntaxOnly constraint by construction.
• Declaration emission: Retain : enum and serialize evaluated member values into .d.ts files. For example, A: 1 << 2 becomes A: 4 only in declaration output; JavaScript retains A: 1 << 2. Under isolatedDeclarations, self-contained constant expressions are supported, while references to symbols outside the enum are rejected, including constants declared in the same file.

For named variable statements annotated with : enum, the checker applies the following rules. These are TypeScript semantic restrictions, not additional validation requirements for type-stripping tools.

• Closes as enum assertion for object literals #60790
• Based on Implement erasable Enum Annotations #61414