Port erlfmt formatting to 3.x

From PR: #3568

A few changes to the formatting logic compared to `main`:

  * rebar3 and `erlfmt` don't work with Erlang 20, so added an OTP version
    check to skip formatting for Erlang 20

  * `make erlfmt-*` commands are faster. Instead of spawning an `erlfmt`
    process per-file, spawn one per `directory/*.erl` pattern.

  * Remove the non-0 return code work-around. `erlfmt` was returning an error
    on `main` because the line width option used for formatting was different
    than the one used during format checking. Making both the default removed
    all the spurious non-0 exits with the current version of `erlfmt`. Thanks
    to Adam Kocoloski for pointing out the issue.

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