Fix for math-related deprecations

[Br1ght0ne]

Because of https://github.com/CleverRaven/Cataclysm-DDA/pull/71286, arithmetic and adjust_var used by this mod no longer work. I attempted to port these to math syntax as best as I could. This is my first encounter with EOCs, so this PR might require revisions.

Incorporates time fixes from https://github.com/TGWeaver/CDDA-Sky-Islands/pull/57 (thanks @bcmcms!)

[Br1ght0ne]

My bad: I did not see #55 and #57 before opening this PR. I included the patch by @bcmcms in this PR, but if any of the previous PRs get merged, I'll update my PR.

[Br1ght0ne]

Needs to be fixed:

errors related to `u_val('missionswon')`

``` DEBUG : error setting double destination in 'u_val: missionswon', FUNCTION : static void kwargs_shim::throw_error(std::string_view) FILE : src/math_parser_shim.h LINE : 53 VERSION : 1949d26 ``` ``` DEBUG : unrecognized number source in 'u_val: missionswon', FUNCTION : static void kwargs_shim::throw_error(std::string_view) FILE : src/math_parser_shim.h LINE : 53 VERSION : 1949d26 ```

[DenCheesecake]

@Br1ght0ne thank you for the update! How critical are the errors related to u_val('missionswon')? Is the current build still playable with these?

[Br1ght0ne]

Thank you @aucun6352!

Re @DenCheesecake: the build was playable, though the "missions won" number in expedition stats was broken. Now, thanks to @aucun6352, everything should work and throw no errors!

[Br1ght0ne]

Handled by https://github.com/CleverRaven/Cataclysm-DDA/pull/71496.