Drop PHP 7 support

[thekid]

Implements last phase of xp-framework/rfc#343. Minimum required PHP version goes from 7.4.0 to 8.0.0!

• Create a three branch for easy backporting
• Composer
• CI pipeline
• README
• Annotation parsing via AST library
• PHP 7 named arguments emulation

This will result in a major release, 4.0.0 at the time of writing!

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Version checks left:

$ grep -Hirn PHP_VERSION src/  | grep -E '[<>]'
src/main/[...]/Constant.class.php:36:      PHP_VERSION_ID >= 80300 ? $this->reflect->getType() : null,
src/main/[...]/Constant.class.php:50:    $t= PHP_VERSION_ID >= 80300 ? $this->reflect->getType() : null;
src/main/[...]/Constructor.class.php:47:        PHP_VERSION_ID < 80100 && $this->reflect->setAccessible(true);
src/main/[...]/Method.class.php:56:      PHP_VERSION_ID < 80100 && $this->reflect->setAccessible(true);
src/main/[...]/Property.class.php:80:      PHP_VERSION_ID < 80100 && $this->reflect->setAccessible(true);
src/main/[...]/Property.class.php:103:      PHP_VERSION_ID < 80100 && $this->reflect->setAccessible(true);
src/main/[...]Type.class.php:34:    if (PHP_VERSION_ID >= 80200) {
src/main/[...]/Type.class.php:176:        PHP_VERSION_ID < 80100 && $reflect->setAccessible(true);
src/test/[...]/VirtualPropertiesTest.class.php:21:    if (PHP_VERSION_ID >= 80100) {