Add some missing keywords for mdBook syntax highlighting (#2903)

Add some missing keywords to the `sway.js` file. This was triggered by
@Braqzen noticing some inconsistent highlighting in the generated MD
books.

Doesn't address custom types not being highlighted but that can be for a
future PR.

scripts/highlightjs/sway.js

@@ -25,19 +25,24 @@ export default function(hljs) {
     "abi",
     "as",
     "asm",
+    "break",
     "const",
+    "continue",
     "contract",
+    "dep",
     "deref",
+    "else",
     "enum",
     "fn",
+    "for",
     "if",
     "impl",
     "let",
     "library",
     "match",
     "mut",
-    "else",
     "predicate",
+    "pub",
     "ref",
     "return",
     "script",
@@ -54,6 +59,10 @@ export default function(hljs) {
   const LITERALS = [
     "true",
     "false",
+    "Some",
+    "None",
+    "Ok",
+    "Err",
   ];
   const BUILTINS = [