Primarily, instead of building a parse pattern enum, we just build the regex
directly, with the appropriate capture group names so that the column name
codepaths can be shared between simple and `--regex` patterns.
Also removed capture group count compared to column name count. I don't think
this codepath can possibly be reached with the regex we now use for the
simplified capture form.
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
