fix(gh_actions_ls): use root_dir and add init_options

[chrisgrieser]

1. Replace root_markers and workspace_required = true with a root_dir function that checks the file is located in the correct location. This prevents some edge cases where the LSP would still attach to non-workflow files. This also makes unnecessary to use a yaml.github filetype, as the readme of the lsp describes.
2. add an empty init_options table, since such a table is apparently needed by the LSP: feat(gh_actions_ls): add vim.lsp.config support #3713 (comment)

[gstokkink]

Could we get a new release for this and other LSP fixes? Been a month since the last release, though I don't know how stable the master branch is at the moment.