feat: add support for ST_SheetState (visibility)

[flhorizon]

Allow parsing and managing the visibility of sheets.

I've tried to be as conservative as possible but perhaps xlSheets would benefit from having it's own ADT rather than expanding sheets tuple?

edit more details:

Although it feels clunkier to extend _xlSheets tuples to include the ST_State, it's a sheet attribute on the same level as the name.

I first though about adding a Worksheet field for the state (and perhaps that would've been better?) but reading how <sheet ... > attributes are handled I've decided to track it at the same level as sheet names

Tell me how you feel about it :) it would simplify _xlSheets lenses a great deal to rather have it as _wsState or something.

[qrilka]

Another problem with microlens, will you take a look?

[flhorizon]

Hello! I've fixed the compilation errors with microlens

[qrilka]

Would you mind resolving the conflict as well?

[flhorizon]

Hey

So I've moved the state to WorkSheet, addressed other comments and added a few related tests