Asynchronous `addSourceFilesAtPaths` and other methods

Right now the methods for adding sources files from the file system are synchronous and the ones for writing back to the file system are asynchronous by default (with sync api available via `-Sync` methods).

This was done because the compiler api reads from the file system synchronously. That said, methods like `addSourceFilesAtPaths`, `addSourceFileAtPath`, etc... could all be asynchronous. `addSourceFilesAtPaths` in particular could get a huge performance boost by parallelizing the work.

The existing api would be moved to `-Sync` methods. For example, `addSourceFilesAtPaths` would become `addSourceFilesAtPathsSync` and then be replaced with an asynchronous `addSourceFilesAtPaths`. The downside to this change is that if someone doesn't read the breaking changes doc and is not inspecting the return value, then they might not notice this has become asynchronous.

Relates to #702.

Start with addSourceFilesAtPaths, addSourceFileAtPath, and the other filesystem source methods described here, then review related issue #702 and the breaking changes documentation. Done means the affected synchronous methods have corresponding -Sync APIs and asynchronous replacements, with the parallelization and compatibility consequences resolved.

Written by the indexing model from the issue text.