Add regexp builder that uses completion-try-completion & co.

[basil-conto]

The default Emacs M-x command automatically runs a function if the input can only resolve to a single function. For example, when running M-x SPC r-st, it will automatically run replace-string, since that is the only function that starts with an r in the first part (before a hyphen) and starts with st after the first hyphen.

With Counsel - tested with make plain - r st completes to rst-mode, and r-st completes to nothing (unless fuzzy search is used). Is there a way to use this hyphen-based searching? Where a hyphen sort of "forces" the command to search for functions with a hyphen in the correct spot? I would also be up for creating a pull request, but am not too used to Lisp, and ivy--regex-plus seems still a bit magic.

Originally posted by @RensOliemans in #2985