build: move all generator scripts to src/gen/

- Move all generator Lua scripts to the `src/gen/`
- Add a `.luarc.json` to `src/gen/`
- Add a `preload.lua` to `src/gen/`
  - Add `src` to `package.path` so it aligns with `.luarc.json'
- Fix all `require` statements in `src/gen/` so they are consistent:
    - `require('scripts.foo')` -> `require('gen.foo')`
    - `require('src.nvim.options')` -> `require('nvim.options')`
    - `require('api.dispatch_deprecated')` -> `require('nvim.api.dispatch_deprecated')`

src/gen/preload_nlua.lua

@@ -0,0 +1,17 @@
+local srcdir = table.remove(arg, 1)
+local nlualib = table.remove(arg, 1)
+local gendir = table.remove(arg, 1)
+
+package.path = (srcdir .. '/src/?.lua;')
+  .. (srcdir .. '/runtime/lua/?.lua;')
+  .. (gendir .. '/?.lua;')
+  .. package.path
+
+_G.vim = require 'vim.shared'
+_G.vim.inspect = require 'vim.inspect'
+package.cpath = package.cpath .. ';' .. nlualib
+require 'nlua0'
+vim.NIL = vim.mpack.NIL -- WOW BOB WOW
+
+arg[0] = table.remove(arg, 1)
+return loadfile(arg[0])()