Add support for jsconfig.json in language service

[zhengbli]

This PR adds support for jsconfig.json in the language service. The implementation treats jsconfig.json as a tsconfig.json file with the allowJs compiler option set to true.

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Edit by @DanielRosenwasser: Fixes #6561.

[mhegazy]

this is going to be an API breaking change. can we add an optional argument for the filename?

[zhengbli]

Done

[billti]

There's a findConfigFile function in program.ts also which only looks for tsconfig.json currently. This appear to only be used by the tsc command-line compiler though, so not needed for this review. Please add an issue to fix it though for the effort to compiler JavaScript at the command line (issue #4792).

[zhengbli]

Opened issue #6561 for further discussion

[zhengbli]

@mhegazy @billti any other comments?

In addition, all error messages suggesting adding a new tsconfig.json should be updated with recommending both tsconfig.json and jsconfig.json. Though those can be a separate PR after 1.8 release.

[billti]

Default "module" to "commonjs" also for jsconfig.json files, as by default we infer CommonJS from require calls or exports assignments, and also this will allow usage of ES6 syntax without getting errors about setting the module type (which is mostly needed for emit, which usually doesn't apply in the .js case).

[billti]

One more change, then 👍