_watchfiles.lua

PR #23689 assumes `client.config.capabilities.workspace.didChangeWatchedFiles`
exists when checking `dynamicRegistration`, but thats's true only if it was
passed to `vim.lsp.start{_client}`.

This caused #23806 (still an issue in v0.9.1; needs manual backport), but #23681
fixed it by defaulting `config.capabilities` to `make_client_capabilities` if
not passed to `vim.lsp.start{_client}`.

However, the bug resurfaces on HEAD if you provide a non-nil `capabilities` to
`vim.lsp.start{_client}` with missing fields (e.g: not made via
`make_client_capabilities`).

From what I see, the spec says such missing fields should be interpreted as an
absence of the capability (including those indicated by missing sub-fields):
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#clientCapabilities

Also, suggest `vim.empty_dict()` for an empty dict in
`:h vim.lsp.start_client()` (`{[vim.type_idx]=vim.types.dictionary}`
no longer works anyway, probably since the cjson switch).