Support the "re2j" regex library

[beckjake]

If you edit /etc/config.properties you can set a regex-library flag to a value of JONI (the default) or RE2J. Setting it to RE2J and doing dbt run results in:

  Runtime Error
    PrestoUserError(type=USER_ERROR, name=INVALID_FUNCTION_ARGUMENT, message="error parsing regexp: invalid escape sequence: `\Z`", query_id=20200403_133843_00004_agst6)

There are some extremely vague docs here and the project github (I assume!) is here. The presto docs are a little incomplete, at best - they don't really make any effort to explain that some valid JONI patterns aren't allowed. The presto page gives this list of caveats. I believe the issue is with Matcher.requireEnd(), but regardless that's quite a list!

I think we should enable RE2J in our test image, because I think RE2J's regex pattern support is a strict subset of JONI's. Then we should fix the bugs, there's at least one!