Manually re-define functions under `function` module with proper type definitions

[lambdalisue]

Functions under the function/_generated.ts module are produced programmatically without type definitions. So we need contributors helps to re-define these manually with proper type definitions like function/buffer.ts.

How to (partially) contribute to this issue

1. Add (or update) a module in function and write proper function definitions
   • Write proper type definitions
   • Write proper document comments
   • Write unit tests for the functions
2. Add the module to _manual.ts if missing
3. Execute make gen to re-generate _generated.ts so that manually defined functions are removed from _generated.ts
4. Create a PR for that and link to this issue

Please use definitions of the following while those are the minimum versions which denops.vim support

• Vim 8.1.2424 eval.txt
• Neovim 0.4.0 eval.txt

[lambdalisue]

• cursor-functions @kuuote