refactor(gwt-mst): apply enumeration to mst package for readability #3897

Job	Run time
Build	1m 48s
Check format	1m 32s
Smoke test	6m 0s
Test Intel fortran (ubuntu-22.04)	40m 38s
Test GNU fortran (macos-13, false)	35m 51s
Test Intel fortran (macos-13)	26m 45s
Test GNU fortran (macos-14, false)	35m 11s
Test Intel fortran (windows-2022)	46m 36s
Test GNU fortran (macos-14, true)	1h 4m 42s
Test GNU fortran (ubuntu-22.04, false)	39m 7s
Extended testing (ubuntu-22.04)	18m 11s
Extended testing (macos-13)	47m 7s
Test GNU fortran (ubuntu-22.04, true)	1h 31m 29s
Extended testing (windows-2022)	46m 5s
Test GNU fortran (windows-2022, false)	45m 56s
	9h 6m 58s

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