refactor(gwt-mst): apply enumeration to mst package for readability #3890

Job	Run time
Build	1m 41s
Check format	1m 13s
Smoke test	5m 59s
Test Intel fortran (ubuntu-22.04)	39m 27s
Extended testing (ubuntu-22.04)	18m 0s
Test Intel fortran (macos-13)	24m 54s
Test GNU fortran (macos-13, false)	38m 20s
Extended testing (macos-13)	52m 6s
Test Intel fortran (windows-2022)	1h 4m 53s
Test GNU fortran (macos-14, false)	33m 15s
Extended testing (windows-2022)	1h 38m 30s
Test GNU fortran (macos-14, true)	1h 5m 18s
Test GNU fortran (ubuntu-22.04, false)	38m 32s
Test GNU fortran (ubuntu-22.04, true)	1h 33m 59s
Test GNU fortran (windows-2022, false)	49m 31s
	10h 25m 38s

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