Add C++20 modules support

[mikomikotaishi]

This pull request adds support for C++20 modules, and is supported using CMake. (Other build systems can be supported in the future.) We can now import the library with:

import re2;

using re2::RE2;

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

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[github-actions]

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