feat(lua): add vim.func._memoize

Memoizes a function, using a custom function to hash the arguments.

Private for now until:

- There are other places in the codebase that could benefit from this
  (e.g. LSP), but might require other changes to accommodate.
- Invalidation of the cache needs to be controllable. Using weak tables
  is an acceptable invalidation policy, but it shouldn't be the only
  one.
- I don't think the story around `hash_fn` is completely thought out. We
  may be able to have a good default hash_fn by hashing each argument,
  so basically a better 'concat'.

runtime/lua/vim/_editor.lua

@@ -29,6 +29,7 @@ for k, v in pairs({
   treesitter = true,
   filetype = true,
   loader = true,
+  func = true,
   F = true,
   lsp = true,
   highlight = true,