Fix word boundary regexes in Ruby grammar (#2984)

### Motivation

As pointed out in #2371 (comment), `\b` is indeed incorrect. All regexes inside the JSON grammar have to be escaped and so word boundary is `\\b`.

### Implementation

Added the missing backslashes.

vscode/grammars/ruby.cson.json

@@ -65,7 +65,7 @@
         }
       },
       "comment": "class Namespace::ClassName < OtherNamespace::OtherClassName",
-      "match": "\b(class)\\s+(([a-zA-Z0-9_]+)((::)[a-zA-Z0-9_]+)*)\\s*((<)\\s*(([a-zA-Z0-9_]+)((::)[a-zA-Z0-9_]+)*))?",
+      "match": "\\b(class)\\s+(([a-zA-Z0-9_]+)((::)[a-zA-Z0-9_]+)*)\\s*((<)\\s*(([a-zA-Z0-9_]+)((::)[a-zA-Z0-9_]+)*))?",
       "name": "meta.class.ruby"
     },
     {
@@ -80,7 +80,7 @@
           "name": "punctuation.separator.namespace.ruby"
         }
       },
-      "match": "\b(module)\\s+(([a-zA-Z0-9_]+)((::)[a-zA-Z0-9_]+)*)",
+      "match": "\\b(module)\\s+(([a-zA-Z0-9_]+)((::)[a-zA-Z0-9_]+)*)",
       "name": "meta.module.ruby"
     },
     {
@@ -92,7 +92,7 @@
           "name": "punctuation.separator.inheritance.ruby"
         }
       },
-      "match": "\b(class)\\s*(<<)\\s*",
+      "match": "\\b(class)\\s*(<<)\\s*",
       "name": "meta.class.ruby"
     },
     {