Rule::noqa_code previously return a single &'static str,
which was possible because we had one enum listing all
rule code prefixes. This commit series will however split up
the RuleCodePrefix enum into several enums ... so we'll end up
with two &'static str ... this commit wraps the return type
of Rule::noqa_code into a newtype so that we can easily change
it to return two &'static str in the 6th commit of this series.
Post this commit series several codes can be mapped to a single rule,
this commit therefore renames Rule::code to Rule::noqa_code,
which is the code that --add-noqa will add to ignore a rule.
The new `ruff rule` output format introduced in
551b810aeb doesn't print Markdown but
rather some rich text with escape sequences for colors and links,
it's actually the "text" format that prints Markdown, so naming the new
format "markdown" is very confusing. This commit therefore renames it to
"pretty".
This isn't a breaking change since there hasn't been a release yet.