Enhancements to SourceMap decoder from tsserver

[sheetalkamat]

Current design for creation of source map decoder per project isn't efficient for tsserver. It could be referencing multiple projects for same declaration source map's and each project creates its own source map decoder. Apart from that each source map decoder reads for files and caches them in the decoder for files that aren't present in the program corresponding to the project.

With this approach, it adds a method on LanguageServiceHost as

• getDocumentPositionMapper: When provided it uses this method to get the document position mapper instead of creating one on its own.
• getDocumentPositionMapper: When provided it gets the SourceFileLike subset to SourceFile that offers text or optionally methods to map line character to absolute position and other way around. (This helps with us just using snapshots when created to transform these instead of recomputing line starts and doing custom operation.

With this new API, tsserver project now implements these apis to create and cache the Document position mapper on the declaration file's script info. This is maintained till there is change in map file/declaration file. Apart from document position mapper, it maintains the list of source script infos associated with it to update the projects from whenever map file changes.

Apart from this change, it also fixes few existing issues found during additional test writing:

• File presence detection in the project where the sourceFile(ts file) is assumed to be part of project and its locations are used in generated file (d.ts) file because program.getSourceFile on both path gives the same file (.d.ts).
• There was also the issue of sourceIndex being compared instead of sourcePosition to sort the sourceMappings based on source location which is fixed in this.
• Missing path included path for source (.ts) file when the project references the generated file(.d.ts) and file is not found.

[sheetalkamat]

Please review the PR so we can get this in soon.

[weswigham]

You should get @rbuckton to look over the sourcemap decoder changes, too.

[sheetalkamat]

Ping @rbuckton

[sheetalkamat]

Merging this. Will handle any feedback that @rbuckton provides in separate PR.