enh(nim) add missing keywords (#4171)

Used the [keyword list from the manual](https://nim-lang.org/docs/manual.html#lexical-analysis-identifiers-amp-keywords) and added the missing ones

Author: ire4ever1190

CHANGES.md

@@ -40,6 +40,7 @@ Core Grammars:
 - fix(nix) don't mix escapes for `"` and `''` strings [h7x4][]
 - fix(swift) - Fixed syntax highlighting for class func/var declarations [guuido]
 - fix(yaml) - Fixed wrong escaping behavior in single quoted strings [guuido]
+- enh(nim) - Add `concept` and `defer` to list of Nim keywords [Jake Leahy]
 
 New Grammars:
 
@@ -83,6 +84,7 @@ CONTRIBUTORS
 [Somya]: https://github.com/somya-05
 [guuido]: https://github.com/guuido
 [clsource]: https://github.com/clsource
+[Jake Leahy]: https://github.com/ire4ever1190
 
 
 ## Version 11.10.0

src/languages/nim.js

@@ -64,9 +64,11 @@ export default function(hljs) {
     "break",
     "case",
     "cast",
+    "concept",
     "const",
     "continue",
     "converter",
+    "defer",
     "discard",
     "distinct",
     "div",