fix(scratchpad): Treat boolean false as a valid return value

[mrjones2014]

Resolves: #420

How to Test

1. Type return false in scratchpad
2. Eval lua
3. Should show returned boolean instead of saying "no return value"

Testing for Regressions

I have tested the following:

• Triggering keymaps from legendary.nvim in all modes (normal, insert, visual)
• Creating keymaps via legendary.nvim, then triggering via the keymap in all modes (normal, insert, visual)
• Triggering commands from legendary.nvim in all modes (normal, insert, visual)
• Creating commands via legendary.nvim, then running the command manually from the command line
• augroup/autocmds created through legendary.nvim work correctly