feat(lsp): vim.lsp.is_enabled() #33703

Problem:
No way to check if a LSP config is enabled without causing it to
resolve. E.g. `vim.lsp.config['…'] ~= nil` will resolve the config,
which could be an unwanted and somewhat expensive side-effect.

Solution:
Introduce `vim.lsp.is_enabled()`.

(cherry picked from commit 03d378fda6)

runtime/lua/vim/lsp.lua

@@ -615,6 +615,14 @@ function lsp.enable(name, enable)
   end
 end
 
+--- Checks if the given LSP config is enabled (globally, not per-buffer).
+---
+--- @param name string Config name
+--- @return boolean
+function lsp.is_enabled(name)
+  return lsp._enabled_configs[name] ~= nil
+end
+
 --- @class vim.lsp.start.Opts
 --- @inlinedoc
 ---