keymaps: fix unable to set noremap = false

Currently setting `vim.keymaps.*.noremap = false` has no effect.
Given that noremap is set by default, the only way to undo it is
to use `remap = true`. This commit adds `remap` as an internal option,
and derives its final value as the inverse of noremap.
This way, setting `noremap` to `false` now behaves as expected.

See https://neovim.io/doc/user/lua-guide.html#_creating-mappings

docs/manual/release-notes/rl-0.9.md

@@ -178,4 +178,8 @@ https://github.com/gorbit99/codewindow.nvim
 - Add [codewindow.nvim] plugin in `vim.assistant.codewindow` with `enable` and
   `setupOpts`
 
+[irobot](https://github.com/irobot):
+
+- Add `remap` option to `vim.keymaps`
+
 <!-- vim: set textwidth=80: -->