feat(lsp): support function for client root_dir (#31630)

If root_dir is a function it is evaluated when the client is created to
determine the root directory.

This enables dynamically determining the root directory based on e.g.
project or directory structure (example: finding a parent Cargo.toml
file that contains "[workspace]" in a Rust project).

runtime/doc/lsp.txt

@@ -683,6 +683,13 @@ Lua module: vim.lsp                                                 *lsp-core*
                          the LSP server will base its workspaceFolders,
                          rootUri, and rootPath on initialization. Unused if
                          `root_dir` is provided.
+      • {root_dir}?      (`string|fun(cb:fun(string))`) Directory where the
+                         LSP server will base its workspaceFolders, rootUri,
+                         and rootPath on initialization. If a function, it
+                         accepts a single callback argument which must be
+                         called with the value of root_dir to use. The LSP
+                         server will not be started until the callback is
+                         called.
       • {reuse_client}?  (`fun(client: vim.lsp.Client, config: vim.lsp.ClientConfig): boolean`)
                          Predicate used to decide if a client should be
                          re-used. Used on all running clients. The default