Update Crystal syntax highlighting

[oprypin]

(In the order of the diff:)

• Add missing "rare" keywords for opening a type, such as enum.
• Remove non-keywords: initialize, it, alias_method, and and others.
• Add missing keywords such as forall.
• Don't highlight keywords right before a colon, these always become named args. Fixes #133.
• Highlight .is_a? and other pseudo-methods as keywords.
• Update the list of builtins, for example, remove delay, add gets.
• Don't highlight builtins if they immediately follow a dot.
• Don't highlight builtins such as class_property as keywords, rather just normal builtins.
• Remove Ruby's %W %S %R %s strings, add %i strings.
• Remove Ruby's arbitrary delimiter strings %OstringO, keep only %|string|.
• Highlight quoted symbols :"sym" same as normal symbols.
• Update the list of regex flags.
• Highlight :[]= symbol just like :[]?.
• Precisely define what kind of escape sequences in a string are valid.