Add support for `enum-string` #309

Job	Run time
Coding Standards / Coding Standards (8.0)	1m 4s
Dependency analysis / Dependency Analysis (8.0)	24s
Code Coverage / Code Coverage	33s
Lint root / Lint yaml & Composer (8.0)	13s
Static analysis / Static Code Analysis (8.0)	44s
Unit test / Supported Versions Matrix	7s
Unit test / Tests (7.4, lowest)	39s
Unit test / Tests (7.4, locked)	28s
Unit test / Tests (8.0, locked)	15s
Unit test / Tests (8.0, highest)	52s
Unit test / Tests (8.1, lowest)	40s
Unit test / Tests (8.1, locked)	33s
Unit test / Tests (8.2, highest)	34s
Unit test / Tests (8.0, lowest)	39s
Unit test / Tests (8.2, locked)	23s
Unit test / Tests (8.2, lowest)	1m 0s
Unit test / Tests (8.1, highest)	36s
Unit test / Tests (7.4, highest)	36s
Unit test / Tests (8.3, highest)	43s
Unit test / Tests (8.5, locked)	1m 1s
Unit test / Tests (8.4, lowest)	50s
Unit test / Tests (8.4, locked)	44s
Unit test / Tests (8.3, locked)	47s
Unit test / Tests (8.5, lowest)	38s
Unit test / Tests (8.3, lowest)	35s
Unit test / Tests (8.5, highest)	39s
Unit test / Tests (8.4, highest)	33s
	16m 50s

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