MatchPattern: allow regex patterns

[ferbar]

Which issue(s) this PR fixes:
Fixes #3057

What this PR does / why we need it:
Allow a (plain) regex as MatchPattern in <filter .... >
With a regex - assertion it allows a pattern that matches a tag not starting with a string. I think the regex MatchPattern can also replace the commented-out carachter classes.

Docs Changes:
Following lines should be added to https://docs.fluentd.org/configuration/config-file#how-match-patterns-work:
Starting with fluentd a match pattern may be a regex by enclosing the regex with slashes.

Regex match patterns

A regex match pattern looks like <filter /regex/>
This allows character classes or match patterns not starting with a string.

# match tags starting with a
<filter a.** >
   ....
</filter
# match all tags not starting with a.
<filter /(?!a\.).*/ >
   ....
</filter>

For a complete list of supported regex expressions see: https://ruby-doc.org/core-2.7.1/Regexp.html

Release Note:
Add regex match patterns

Things I've considered:

• The tag is checked with @regex.match(tag-name). I don't think that there's a way to break something with a bad regex.
• This patch wraps the regex with \A and \Z. I think this would be a more convenient behavior than without it

[repeatedly]

Looks good.

Put comments for better ruby code.

[repeatedly]

pat.start_with?('/') and pat.end_with?('/') are better.

[repeatedly]

pat[1..-2] is better

[repeatedly]

Thanks!