Add C++20 modules support

[mikomikotaishi]

This PR adds C++20 modules support. No existing issue covers this pull request. The module generated is re2 and enabled using the CMake option RE2_BUILD_MODULES.

Example:

import re2;

using re2::RE2;

// ...
String pattern = "h.*o";
String input = "hello";
bool match = RE2::FullMatch(input, pattern);

Build using:

cmake -S . -B build -G Ninja -DRE2_MODULES=ON
cmake --build build

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