Problem:
`nvim_set_option_value` cannot "update" options similar to `:set opt=`,
`:set opt+=`, etc. The Lua impls of "vim.opt" / "vim.o" have incomplete,
bespoke reimplementations of those operations.
ref #38420
Solution:
- Add `operation` param to `nvim_set_option_value`, which may be "set",
"append", "prepend", or "remove".
- Use this feature to implement `vim.opt` / `vim.o`.