refactor(codemode): route transpilation through a #transpile conditional import

[kitlangton]

What / Why

Runtimes bundled with the workerd condition cannot carry the TypeScript transpiler dependency: the compiler is large and probes Node internals during module initialization.

Route interpreter transpilation through a #transpile conditional import. Node and Bun resolve the extracted TypeScript implementation, preserving the existing path byte-for-byte, while workerd bundles resolve an inert variant that passes JavaScript through without importing the compiler. TypeScript-only syntax then fails at the interpreter parse boundary with a clear parse error.

This follows the #global-roots conditional-import precedent in packages/util from #41918.

Scope

• Adds the #transpile package imports map in packages/codemode.
• Extracts the existing Node/Bun transpilation implementation without changing its compiler options or diagnostic formatting.
• Adds the workerd pass-through implementation.
• Does not add TypeScript transpilation support to workerd.

Testing

• bun run typecheck in packages/codemode
• bun run test in packages/codemode (1,089 passing)
• Push hook: bun turbo typecheck --concurrency=3 (32 packages passing)