Provide deno check as runtime API

[jsejcksn]

Deno.emit has come and gone, and plenty of suggestions can be found pointing toward using esbuild to transpile TypeScript source code at runtime, but there is now a lack of a runtime API for type-checking.

Previously, with Deno.emit, type-related diagnostics were available for determining if transpiled source code had type-related problems. Now, there is no way to know if the transpiled source is unsound from a type system perspective.

It would be very useful for Deno to provide runtime API access to its type-checking functionality and diagnostics. It's not possible to use tsc or tools in its ecosystem for this purpose (primarily because Deno's import specifier syntax is incompatible).

I believe this should be provided directly from the runtime on the Deno namespace (instead of as an external module like https://github.com/denoland/deno_emit) because the modified version of the compiler that is needed for this task is already so tightly-integrated. Deno saliently advertises TypeScript integration as one of its core features (home page, introduction in the manual, etc.). I find it unexpected for such a platform not to provide TypeScript runtime integration.

Related:

• Public API for compilers #1739
• Provide deno fmt and deno lint as runtime APIs #10731
• Query for potentially-related open issues: check typescript api