Update rust syntax: char literal (#2162)

Highlight character literals started with a single quote (').
Importantly this ensures correct highlighting for the character literal '"'.
Limitation: rust char literals contain exactly one character, however this isn't checked by the highlighter.

Closes #2160

runtime/syntax/rust.yaml

@@ -24,15 +24,23 @@ rules:
     - constant.string:
         start: "\""
         end: "\""
-        skip: "\\\\."
+        skip: '\\.'
         rules:
-            - constant.specialChar: "\\\\."
+            - constant.specialChar: '\\.'
 
     - constant.string:
         start: "r#+\""
         end: "\"#+"
         rules: []
 
+    # Character literals
+    - constant.string:
+        start: "'"
+        end: "'"
+        skip: '\\.'
+        rules:
+            - constant.specialChar: '\\.'
+
     - comment:
         start: "//"
         end: "$"
@@ -49,4 +57,3 @@ rules:
         start: "#!\\["
         end: "\\]"
         rules: []
-