feat(stringvalidator): client provided regex

[mttrbrts]

This PR reverts the changes introduced in #484 to adopt the RE2 engine. The engine has some constraints (no browser support, memory limits etc.), so I'd like to simplify Concerto, but allow consumers to supply their own engine where they can make own judgements on the tradeoffs.

Changes

• Revert back to the default JavaScript RegExp engine by default for all platforms.
• Add a new options parameter to the ModelManager constructor to allow a client to provide their own RegExp engine.

const XRegExp = require('xregexp')
XRegExp.install('astral');
const modelManager = new ModelManager({ regExp: XRegExp });

Flags

• This change makes Concerto vulnerable to ReDoS attacks if client applications do not introduce further checks. This strategy is consistent with the approach that other validation libraries take (see AJV).

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to master from fork:branchname

[sstone1]

LGTM