Runners in language module

[bquorning]

Extract a named Language constant from the duplicated {:to :not_to :to_not}/{:to :to_not :not_to} in our node matchers.

Language::Runners is extracted from #726.

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[pirj]

Looks good 👍
Will update my pull request once this is merged.

[Darhazer]

I was wondering how those should be called.