neovim: Convert all config to Lua

I've been using this with only minimal changes for a few weeks now, so
this seems like a good place to checkpoint it.

neovim/colors/jdkaplan-warm.lua

@@ -0,0 +1,14 @@
+vim.opt.background = 'light'
+vim.g.colors_name = 'jdkaplan-warm'
+
+vim.o.termguicolors = true
+
+local pkg = 'jdkaplan.warm_theme'
+
+-- Clear the Lua cache for the theme package to force it to load every time.
+package.loaded[pkg] = nil
+
+local lush = require('lush')
+local theme = require(pkg)
+
+lush(theme)