Add "single file mode"

[artempyanykh]

Currently, Marksman requires a project/root/folder to provide language features such as cross-file references, workspace symbols, and rename refactor that works across many files.

However, it would be convenient to provide a subset of language features in situations where there's no project/root/folder. Some features like "document symbols" will be the same as before in "single-file mode". Other will lose cross-file capabilities and will work only intra-file, e.g. goto def, references, renames, completion. But perhaps this is enough for one-off edits of random markdown files.

[artempyanykh]

/cc @jwortmann @jfcherng @rwols this is a continuation of #66 focused on "single file mode" support.

[jfcherng]

Thank you @artempyanykh . It works. 👍