This resurrects the `--unstable-detect-cjs` flag (which became stable),
and repurposes it to attempt loading .js/.jsx/.ts/.tsx files as CJS in
the following additional scenarios:
1. There is no package.json
1. There is a package.json without a "type" field
Also cleans up the implementation of this in the LSP a lot by hanging
`resolution_mode()` off `Document` (didn't think about doing that until
now).
