Fix `plugin-types` generation

[fritzmg]

Currently CSPBuilder::allowPluginType does not work at all. There are two issues:

1. compileSubgroup tries to compile the allowed plugin types from the allow sub-key. However that key does not exist, leading to

   1) ParagonIE\CSPBuilderTest\BasicTest::testAllowPluginType
   Undefined array key "allow"
   
   csp-builder\src\CSPBuilder.php:875
   csp-builder\src\CSPBuilder.php:131

   The allowed plugin types are actually stored in the types sub-key.

2. The preg_replace in enc will actually remove any valid mime type string.

This PR implements the following changes:

1. The allowed plugin types are now retrieved from the correct array sub-key.
2. The enc method for mime strings is changed to a preg_match method that will return a valid mime-type definition if found and an empty string if not.
3. The compileSubgroup will return an empty string if no valid plugin type was provided.