Fixed named backreferences in strict mode

[RunDevelopment]

This fixes #23.

Changes:

• countCapturingParens now counts the number of named and unnamed groups separately.
• The _nFlag property is now set by consumePattern instead of validatePattern.
• Patterns are now validated in a single pass (after the initial counting of all capturing groups).
• Added two new test files to verify the fix. The regexes in these test files are the ones from test/fixtures/parser/literal/named-capturing-group-valid-2018.json. I simply copied the file and divided the patterns based on whether they are valid with strict: true.