feat: configurable code block background inset

## Details

Request: https://github.com/MeanderingProgrammer/render-markdown.nvim/issues/643

Adds `code.background_inset` option that defines how far from the start
and end of a code block background rendering should start. The default
value of 1 keeps the existing behavior.

By setting the value to 0 you can extend the background highlight logic
to also include the delimitter lines. This interferes with `code.border`
and `code.language` behavior so should only really be done if both of
those are disabled.