Initial support for module: node12

[weswigham]

This PR adds the basic framework for (compiler options and defaults) and core emit behavior for a new module: node12 and module: nodenext compiler option. Both should be almost identical, for now.

The core of this PR is allowing .js and .ts files to be interpreted as either esm or cjs format files, within the same compilation.

• The esm emit branch is identical to es2020+ module emit, except we transpile TS-style import = require statements down to a helper-like createRequire call, rather than error'ing and eliding them. (This gives easy access to sync require statements as-needed for, eg, conditional top-level access to cjs dependencies)
• The cjs emit branch modifies normal cjs emit to preserve import call expressions as-is, this way they can be used to pull in async (esm) dependencies within cjs-targeted files, and is otherwise normal cjs output.
• Since we don't currently actually read in non-ts and non-js extensions (yet - that'll be a followup pull request), the mode of the .ts or .js file is determined by the enclosing package.json and its type field (if it is present and set to module, .ts and .js files are esm format, otherwise they are cjs format).

As far as checking goes, both options imply esModuleInterop for now (this only really affects default import checking and cjs module emit and may be subject to change when more checker behaviors are added later if they pickup the work this flag does). Additionally, module: node12 implies a moduleResolution: node12 and target: es2020, and module: nodenext implies a moduleResolution: nodenext and target: esnext. The new moduleResolution modes currently just redirect to the old node resolution mode until they're actually implemented in another PR.

Also included is a driveby fix for js declaration emit where export {default} from "fs"; in js was mistakenly serialized to export {"fs" as default} from "fs"; in a declaration file when "fs" is just a shorthand module declaration.