Add keyword for metadata attributes.

Author: whitequark

package.json

@@ -6,7 +6,7 @@
     "author": "Catherine <[email protected]>",
     "license": "BSD-0-Clause",
     "homepage": "https://github.com/prjunnamed/",
-    "version": "0.4.0",
+    "version": "0.4.1",
     "repository": {
         "type": "git",
         "url": "https://github.com/prjunnamed/vscode-syntax.git"

syntaxes/prjunnamed.tmLanguage.json

@@ -41,7 +41,7 @@
       "patterns": [{"include": "#string-escape"}]
     },
     "keyword": {
-      "match": "source|scope|ident|set|buf|not|and|or|xor|mux|adc|eq|ult|slt|shl|ushr|sshr|xshr|mul|udiv|umod|sdiv_trunc|sdiv_floor|smod_trunc|smod_floor|match|assign|dff|iobuf|target|param|input|output|io|name|debug",
+      "match": "source|scope|ident|attr|set|buf|not|and|or|xor|mux|adc|eq|ult|slt|shl|ushr|sshr|xshr|mul|udiv|umod|sdiv_trunc|sdiv_floor|smod_trunc|smod_floor|match|assign|dff|iobuf|target|param|input|output|io|name|debug",
       "name": "keyword.other.prjunnamed"
     },
     "metadata-ref": {