[misc] add readonly typescript interface

[forivall]

Fixes #4169

The same could also be done for Duration but with only 4 mutation methods, it's easier to keep track of when you're modifying.

[marwahaha]

I don't use TypeScript often anymore; perhaps someone else can review this.

[coveralls]

Coverage increased (+0.06%) to 88.499% when pulling 84c3e17 on forivall:feat/readonly-typings into e968092 on moment:develop.

[forivall]

the CI errors are from prettier on files i didn't modify? weird.

[FaizzJamaludin]

#5840

[forivall]

@marwahaha this has 2 approving reviews; someone with merge permissions could probably merge? thank you!

[ichernev]

@forivall you also need to change moment.d.ts and do the Duration as well (otherwise it's inconsistent).