"LspStart! foo" should force attach "foo" client to current buffer

[justinmk]

Problem

No way to force a lsp client to attach to current buffer (or if there is, please let me know).

#2708 proposed introducing a "shebang" flag to force lsp configs for some special cases, but ultimately, if we can't figure out the correct configs automatically, the general solution is to allow users to easily say what to apply (interactively).

Proposal

• :LspStart! foo should force the "foo" config for the current buffer.
  • This may also imply "single file mode", I'm not sure. Especially if a "root" couldn't be found.