Improve formatter support for insertion snippets

[pokey]

After discussion with @AndreasArvidsson, we're inclined to use Talon formatters instead. There are two ways we thought of to do this:

• Could use a knausj comma-separated formatter string in this field, and then Talon would query Cursorless to figure out which formatter to use and do the formatting itself
• Could drop this field here, instead adding a column to the Talon csv that indicates which type of formatter to use. It will support things like user.code_public_function_formatter so that it can be language-dependant. The problem with this approach is that it will be hard to support cross-split insertion once we support that

Originally posted by @pokey in #304 (comment)