[Nerixyz]

This does a few things. I'll split up the different CI runners.

• Treat tests as a subproject and require PAJLADA_SETTINGS_BUILD_TESTS to be set.
• I tried to build with clang-19 and it failed. So I updated the rapidjson that we use with conan. Specifically, we want Tencent/rapidjson@862c39b. Note: We don't use rapidjson 1.1.0 in Chatterino either, but https://github.com/Tencent/rapidjson/tree/d87b698d0fcc10a5f632ecbc80a9cb2a8fa094a5. We should consider using conan for all builds or adding rapidjson as a submodule (which would duplicate it in Chatterino).
• Moved from conanfile.txt to conanfile.py in order to be able to disable CMakeUserPresets.json generation.
• conan_provider.cmake (or conan.cmake before) was already in this repo, so I updated it. This makes it easier to use conan.
• With the conan provider, it makes sense to use CMake presets, so one can just cmake -B build --preset debug-conan and it does the right thing.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.56%. Comparing base (441b528) to head (8ac139f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   72.56%   72.56%           
=======================================
  Files          11       11           
  Lines         492      492           
  Branches       79       79           
=======================================
  Hits          357      357           
  Misses         92       92           
  Partials       43       43           

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