added support for expanding null-ls sources

[WilsonOh]

Hi everyone, I wanted to see the individual null-ls sources as part of the lsp_name section so I tried adding it in. The expansion can be activated by setting the expand_null_ls key to true in the defaults table during setup. i.e.

require('staline').setup {
  defaults = {
    expand_null_ls = true,
    ...
  }
}

With expand_null_ls set to true:

With expand_null_ls set to false:

_The 'LSP: ' part is not part of the lspname section

It can be a bit messy if there are too many null-ls sources but I think it can be made better by filtering out the sources by their methods (diagnostics, formatting etc.).

What do you guys think?

[tamton-aquib]

Epic work, thx for the PR 💯