textrules-move Move "rules" to "syntaxrules" in preparation for adding textrules

[DaveMcEwan]

This branch textrules-move only moves around existing code, and exists only to separate out the real changes to implement textrules.
This shouldn't be merged until the dependent branch textrules is ready (it doesn't exist yet).

Note, this branch contains most of the moving changes, but there's still a few (that I forgot about) on the textrules branch.

See also:

• Discussion on plan for textrules functionality Feature: Extend rules mechanism to further levels of abstraction (long term). #237
• Discussion on this moving Dev Question: New class of rules, textrules, working on lines instead of syntax. #244
• Matching draft PR on svlint-plugin-sample textrules Rename Rule -> SyntaxRule. svlint-plugin-sample#3