Improve support for custom token prefixes. #627

Job	Run time
cs-stan / Coding Standard & Static Analysis	34s
testsuite / testsuite-linux (8.1, highest, false)	42s
testsuite / testsuite-linux (8.2, highest, false)	38s
testsuite / testsuite-linux (8.3, highest, false)	41s
testsuite / testsuite-linux (8.4, highest, false)	1m 25s
testsuite / testsuite-linux (8.1, lowest, false)	28s
	4m 28s

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