Add Regex Engine to FEX for config option loading

[badumbatish]

Hi there, i'm opening the PR to add a small regex engine to the Fex codebase. The motivation is to help dynamic modify some config option based on the name of the config file. Please see the following screenshot for more context.
.

The regex engine is an NFA based one, implemented from the dragon book 2nd ed.

Implementation is in FEXCore/Source/Utils/Regex.cpp.
Test case is unittests/APITests/Regex.cpp.

I took the liberty to refactor and reuse a bit of the code in parsing the json. The traversing of a json list of pair of strings is now a helper function called ListApplier that accepts a json_t.

The default behavior is that the config loader goes through each regex and see if it matches the config name, if it does, it applies the config option specified and then stops checking anymore regex (this can change based on the need and reviews of the PR).

I'm not sure where should i add the test case for the new config loader. I tested locally via /home/ubuntu/.fex-emu/Config.json and it works, would love some suggestions

{
  "Config":
    {"RootFS":"Ubuntu_22_04"},
  "ThunksDB":{},
  "RegexConfig" : {
    ".*" : {
      "RootFS":"Ubuntu_24_04",
    }
  }
}

[bylaws]

I'm unsure on a full regex engine, I think a very naive glob impl might be enough. albeit this is very neat

[Sonicadvance1]

I have a concern about including this regex_engine that has no license. When it says "Inspiration taken from ..." and it's pretty much the code from https://github.com/0xsh4dy/regex_engine .
Attribution doesn't really matter when no license exists.

[Sonicadvance1]

Other than the concern about the licensing of the regex engine used, I'm fine with this type of change.