Add default keymap descriptions

[ryanlaws]

Add default descriptions for keymaps when not specified in keymap configuration:

• map()
• map_expr()

[davidgranstrom]

@ryanlaws Thanks, looks useful!

If you have luacheck and stylua installed locally you can format your PR by running make format in the root of the scnvim repo (running make will run the linters as a default).

To run the unit tests run make test in the root of the repo. The test map_spec.lua should be extended and updated to match the new behaviour introduced by this PR.

Please let me know if there are any issues.

[ryanlaws]

This is awesome - I haven't worked with linting or tests in Lua before. My bad for not seeing them. I fixed the existing test, added a new one for map_expr, and linted it all.

Let me know if I missed anything else. Thank you!

[davidgranstrom]

Looks great, thanks!