Print pcre2test output in colour.

[IsaacOscar]

This adds a -c option to pcre2test that will print various different output in colour.

For ease of reviewing I've split the changes into two commits:

1. contains the interesting changes, but it doesn't compile without the second commit.
2. simply changes all remaining printf, pchars, etc. calls to also include the colour to use.

Most output from pcre2test is given an explicit colour, however this is often just clr_output which is the terminals default colour (i.e. it likely won't be in "colour").

[NWilson]

This is great! I have a few requests:

• I think I prefer having the output FILE* be explicit, and only output to stdout if it's not specified. This matches the C convention - printf goes to stdout, and all the other methods (fprintf etc) have an explicit output destination. This basically means adding "outfile" as an argument to your cprintf throughout. Thoughts?
• I'd prefer --color={always,auto,never} instead of the confusingly-short -c. (I'm happy to add auto later myself. If it were to be the default, it would need to check isatty and getenv("NO_COLOR") per https://no-color.org/.)
• I'm not sure about the valgrind errors being yellow. Speculatively switching to yellow just in case anything else is printed seems a little eager. Why not just restore standard white for anything that doesn't need an explicit colour.

[NWilson]

I've made those changes. I'll merge it soon, unless you message to say there are additional changes you'd like to make.

Thank you very much Isaac!