Add ECMA definition of symmetry operation regexp and associated tests

[ml-evs]

Closes #487.

This PR adds an explicit and simplified symmetry operation regexp that can be used with JSON Schema.
It is validated against the same test cases as the PCRE version with an exact match in the results.

[ml-evs]

Have tagged reviewers but no pressure to look at this before 2024!

[sauliusg]

Looks good to me in principle. As details go, I suggest adding a test that compares the original PCRE REs and the ECMA REs, by generating automatically the ECMA RE from the PCRE by interpolating variables (should be easy to do in Perl) and comparing them with the ECMA RE provided in the specification. In this way we would know that if tests pass, both sets of REs define the same thing.

[rartino]

I've added the 'requires-discussion' label in preparation of the web meet, as it seems we may want to generally find a consensus around the 'pattern' field. (I suppose I could have added the label on #490 instead, but technically it is the possible reference to that appendix from here that is the topic to discuss).

[sauliusg]

Cool test 'symops_pcre_to_ecma_001.sh', I think it is sufficient to safeguard that ECMA and PCER symop definitions do not diverge. Well done!