add 'set_cost' and 'set_sell_cost' contexts

[vitellaryjr]

these contexts can be used to create effects that alter the cost and sell value of cards dynamically. needs to be 2 contexts, so that the original sell value can be based on the new cost, while also allowing new sell values to be based on the original sell value. 'tis a little ugly (and will be mildly annoying to document in a concise way), but it's the best way i could find to do it and make it sufficiently flexible

Additional Info:

• I didn't modify api's or I've made a PR to the wiki repo.
  • contexts need to be documented later
• I didn't modify api's or I've updated lsp definitions.
• I didn't make new lovely files or all new lovely files have appropriate priority.

[VivianGiacobbi]

Similarly to other calculation keys, it might be preferred to send the card's current/expected cost in the context and then update the value in the context via SMODS.update_context_flags(), since currently any consequent calculation returns will simply override the value rather than modify it afaik

[vitellaryjr]

aye, good call; have done that now