feat: make definition a supertype and add string content

[amaanq]

Problem

• The definition rule not being queryable because it's hidden and not listed in the supertypes field
• The string rule not having a named node for the content of it

Solution

• Unhide definition and move it to the supertypes field
• Add a string_content field to string so that one may query for the content only

This PR solves these two problems in a non-breaking manner - all existing queries will function as is with no visible changes/effects