lua: simple snippet support in the completion items (#12118)

Old behavior is: foo(${placeholder: bar, ...)

with lots of random garbage you'd never want inserted.

New behavior is: foo(bar, baz) 

(which maybe is good, maybe is bad [depends on user], but definitely better than it was).

-----

* Implement rudimentary snippet parsing

Add support for parsing and discarding snippet tokens from the completion items.

Fixes #11982

* Enable snippet support

* Functional tests for snippet parsing

Add simplified real-world snippet text examples to the completion items
test

* Add a test for nested snippet tokens

* Remove TODO comment

* Return the unmodified item if the format is plain text

* Add a plain text completion item

runtime/lua/vim/lsp/protocol.lua

@@ -633,8 +633,7 @@ function protocol.make_client_capabilities()
         dynamicRegistration = false;
         completionItem = {
 
-          -- TODO(tjdevries): Is it possible to implement this in plain lua?
-          snippetSupport = false;
+          snippetSupport = true;
           commitCharactersSupport = false;
           preselectSupport = false;
           deprecatedSupport = false;