MEDIUM: regex: pcre2 support

MEDIUM: regex: pcre2 support

this adds a support of the newest pcre2 library,
more secure than its older sibling in a cost of a
more complex API.
It works pretty similarly to pcre's part to keep
the overall change smooth,  except :

- we define the string class supported at compile time.
- after matching the ovec data is properly sized, althought
we do not take advantage of it here.
- the lack of jit support is treated less 'dramatically'
as pcre2_jit_compile in this case is 'no-op'.