Add LTeX extension (#1222)

LTeX LSP implements a language server
according to the Language Server Protocol (LSP)
and provides grammar and spelling errors
in markup documents: LATEX, Markdown, etc..
The documents are checked with LanguageTool.

This is how it works in action:

![CleanShot 2024-08-08 at 21 54
29@2x](https://github.com/user-attachments/assets/3318ef47-5cda-4209-8c18-6711244c349e)
![CleanShot 2024-08-08 at 21 55
40@2x](https://github.com/user-attachments/assets/80789265-7faa-49b8-a7c6-3a08cdf960b6)


See also:
- https://github.com/valentjn/ltex-ls

.gitmodules

@@ -458,6 +458,10 @@
 	path = extensions/lox
 	url = https://github.com/arian81/zed-lox.git
 
+[submodule "extensions/ltex"]
+	path = extensions/ltex
+	url = https://github.com/vitallium/zed-ltex.git
+
 [submodule "extensions/luau"]
 	path = extensions/luau
 	url = https://github.com/4teapo/zed-luau

extensions.toml

@@ -524,6 +524,10 @@ version = "0.0.3"
 submodule = "extensions/lox"
 version = "0.0.1"
 
+[ltex]
+submodule = "extensions/ltex"
+version = "0.0.1"
+
 [lua]
 submodule = "extensions/zed"
 path = "extensions/lua"