Add snippet crate

Co-Authored-By: Nathan Sobo <nathan@zed.dev>
Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/snippet/grammar/grammar.js

@@ -0,0 +1,26 @@
+module.exports = grammar({
+    name: 'snippet',
+
+    rules: {
+        snippet: $ => repeat1($._any),
+
+        _any: $ => choice(
+            $.tabstop,
+            $.placeholder,
+            $.text
+        ),
+
+        tabstop: $ => choice(
+            seq('$', $.int),
+            seq('${', $.int, '}'),
+        ),
+
+        placeholder: $ => seq('${', $.int, ':', $.snippet, '}'),
+
+        int: $ => /[0-9]+/,
+
+        text: $ => choice($._raw_curly, $._plain_text),
+        _raw_curly: $ => token(prec(-1, /}+/)),
+        _plain_text: $ => /([^$}]|\\[$\\}])+/,
+    }
+})