Use readonly text buffers when jumping to dependency sources

[laughedelic]

Use readonly text buffers when jumping to definitions from dependencies. See scalameta/metals#36.

This requires a change in Metals first so that it will send custom metals: URIs for locations in definitions (instead of the current workaround with temporary files). Then on this side there are 2 things to be done:

• Register an opener for custom URIs, which will read the source and
• Create read-only buffer with the content of the source.
  Read-only buffers exist since Atom 1.24, but are currently undocumented. Still, I see setReadonly method in sources, so I assumed it's supported.