Fixes ghostty-org#5257

Specify environment variables to pass to commands launched in a terminal
surface. The format is `env=KEY=VALUE`.

`env = foo=bar`
`env = bar=baz`

Setting `env` to an empty string will reset the entire map to default
(empty).

`env =`

Setting a key to an empty string will remove that particular key and
corresponding value from the map.

`env = foo=bar`
`env = foo=`

will result in `foo` not being passed to the launched commands.
Setting a key multiple times will overwrite previous entries.

`env = foo=bar`
`env = foo=baz`

will result in `foo=baz` being passed to the launched commands.

These environment variables _will not_ be passed to commands run by Ghostty
for other purposes, like `open` or `xdg-open` used to open URLs in your
browser.