Bump to 2.0.0 and add a version-triggered release pipeline

[esrrhs]

Summary

• Bump the public header version macros to 2.0.0 (FAKELUA_VERSION_* / FAKELUA_VERSION_STRING).
• Add a Release workflow that, on master, compares FAKELUA_VERSION_STRING with the previous commit and, if it changed and vX.Y.Z does not already exist, builds the shared library on Linux, macOS, and Windows, then publishes header + dynamic library packages to a GitHub Release.
• Add FAKELUA_BUILD_TESTS / FAKELUA_BUILD_BENCHMARKS CMake options (default ON) so the release job can skip Lua, tests, and benchmarks.

Test plan

• Confirm CI on this PR still builds and tests as before (options default ON).
• After merge to master, confirm the Release workflow detects 0.1.0 → 2.0.0 and creates tag v2.0.0.
• Confirm the three artifacts contain include/fakelua.h plus libfakelua.so / libfakelua.dylib / libfakelua.dll.
• Optional: run Release via workflow_dispatch only if a tag was not created.

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