The use of var and let instead of const is discouraged as specified in the coding conventions.
unfortunately there are so many `var`s and `let`s in rmgen that using a linter while working on maps basically highlight all half of the lines, which is annoying and not having a linter enabled often leads to forgotten `var, let -> cost` changes in the touched lines (at least for me).
This diff does most of the conversion in one go, specifically the more trivially exchangeable cases.
Notes:
alpine_lakes and alpine_valleys have some vars that should be extracted into a biome.
I left most of latium and unknown untouched, since I can't follow easily what their code does.