Extract lua language support into an extension (#10437)

Release Notes:

- Extracted lua language support into an extension, and improved Lua
highlighting and completion label styling.

---------

Co-authored-by: Marshall <marshall@zed.dev>

extensions/lua/languages/lua/config.toml

@@ -0,0 +1,13 @@
+name = "Lua"
+grammar = "lua"
+path_suffixes = ["lua"]
+line_comments = ["-- "]
+autoclose_before = ";:.,=}])>"
+brackets = [
+    { start = "{", end = "}", close = true, newline = true },
+    { start = "[", end = "]", close = true, newline = true },
+    { start = "(", end = ")", close = true, newline = true },
+    { start = "\"", end = "\"", close = true, newline = false, not_in = ["string"] },
+    { start = "'", end = "'", close = false, newline = false, not_in = ["string"] },
+]
+collapsed_placeholder = "--[ ... ]--"