feat(func): allow manual cache invalidation for _memoize

This commit also adds some tests for the existing memoization
functionality.

runtime/lua/vim/func.lua

@@ -3,9 +3,6 @@ local M = {}
 -- TODO(lewis6991): 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` is completely thought out. We
 --   may be able to have a good default hash by hashing each argument,
 --   so basically a better 'concat'.
@@ -17,6 +14,10 @@ local M = {}
 --- Internally uses a |lua-weaktable| to cache the results of {fn} meaning the
 --- cache will be invalidated whenever Lua does garbage collection.
 ---
+--- The cache can also be manually invalidated by calling `:clear()` on the returned object.
+--- Calling this function with no arguments clears the entire cache; otherwise, the arguments will
+--- be interpreted as function inputs, and only the cache entry at their hash will be cleared.
+---
 --- The memoized function returns shared references so be wary about
 --- mutating return values.
 ---
@@ -32,11 +33,12 @@ local M = {}
 ---     first n arguments passed to {fn}.
 ---
 --- @param fn F Function to memoize.
---- @param strong? boolean Do not use a weak table
+--- @param weak? boolean Use a weak table (default `true`)
 --- @return F # Memoized version of {fn}
 --- @nodoc
-function M._memoize(hash, fn, strong)
-  return require('vim.func._memoize')(hash, fn, strong)
+function M._memoize(hash, fn, weak)
+  -- this is wrapped in a function to lazily require the module
+  return require('vim.func._memoize')(hash, fn, weak)
 end
 
 return M