Nudge new users towards using this style:
```lua
local config = {}
config.color_scheme = 'Batman'
return config
```
and surface how to write lua modules closer to the main section
on config files. In that lua modules section, nudge towards using
a convention similar to that of the plugin spec described in
this commit: e4ae8a844d
I've had a couple of people report issues like
https://github.com/wez/wezterm/issues/2378 recently, and we can avoid
this class of problem by unsetting SHELL at proces startup, so that's
what we do.
Since the original code was written, Rust grew and stabilized
an interface for specifying argv0 for the child process, so
we can remove the fragile `-l` argument passing.
refs: https://github.com/wez/wezterm/issues/753
