Making ssplit-cpp self contained for bergamot project

[abhi-agg]

Currently, users are required to have pcre2 library already installed on the system to be able to use ssplit-cpp in bergamot project. To be able to run it for wasm, we need the pcre2 library that is compiled for WASM and link it.

There are 2 ways to solve this issue:

1. Either link against a pcre2 library that is compiled for wasm (it is a bad way of intergrating)
2. Include pcre2 sources in ssplit-cpp in 3rd_party folder, build it and link it in ssplit-cpp as an alternative to linking with pre-installed pcre2 on system

2nd way is clearly more maintainable and clean solution.

I am ready to do this task and can submit a PR.
If @ugermann doesn't want to always compile pcre2 from sources then I am fine with that as well. I can add a cmake option USE_INTERNAL_PCRE2 which can enable/disable building pcre2 from sources.

This is blocking my work and we need to agree fast.

[kpu]

If you're volunteering, I will agree with option 2.

Also, actual question here: why is linking a bad way of integrating? I mean we link with libc right?

[abhi-agg]

@kpu Option 2 done already via this PR. Just waiting for the merge.
@ugermann Can you please merge it?

[abhi-agg]

Closing this as #45 is merged to main.