Support for path completions

[celbig]

Unless I'm mistaken, at the moment, langagueserver does not support (relative) path completion inside charcter strings. There may exists other completion provider that have this options for some some editors but they are not as context aware as an lsp server and may not support relative path (for instance path Intellisense for vs code cannot suggest relative path from the root of the project).

I would like to suggest adding the following feature:

• Suggest a path completion when inside character string either when it is empty or when the part berfore the cursor matches the start of an existing path

I'm going to work on it for myself. So if you think this is a relevant feature, I will submit a PR.