Use shlex.quote for shell argument escaping on POSIX

quote_arg() only wrapped arguments containing spaces in double quotes.
Shell metacharacters ($, backtick, ;, |, &, etc.) were left unescaped.

On POSIX, use shlex.quote() which single-quotes to prevent all shell
interpretation. Retain original Windows behavior since cmd.exe does not
interpret single-quoted strings.

src/solidlsp/util/subprocess_util.py

@@ -1,4 +1,5 @@
 import platform
+import shlex
 import subprocess
 
 
@@ -15,8 +16,14 @@ def subprocess_kwargs() -> dict:
 
 def quote_arg(arg: str) -> str:
     """
-    Adds quotes around an argument if it contains spaces.
+    Quotes a shell argument to prevent interpretation of metacharacters.
+
+    Uses :func:`shlex.quote` on POSIX systems for proper escaping of all
+    shell-special characters. On Windows, wraps arguments containing spaces
+    in double quotes (Windows shell does not interpret single-quoted strings).
     """
-    if " " not in arg:
-        return arg
-    return f'"{arg}"'
+    if platform.system() == "Windows":
+        if " " not in arg:
+            return arg
+        return f'"{arg}"'
+    return shlex.quote(arg)