Support commands list on consumer project (Remove commands list from theme)

The setup we have used for syntax highlighting, command line linker, ...etc. is kind of hard-coded onto the theme but it would be better if it is based on the individual consumer projects. We need to generalize this so that the required data will be supplied from the consumer project and the theme will handle only the logical portion of it.

_Originally posted by @yathomasi in https://github.com/iterative/gatsby-theme-iterative/pull/128#issue-1451846507_

Start by tracing where the theme currently defines syntax-highlighting and command-line linker data, then identify how a consumer project supplies its configuration. Confirm the boundary between consumer-provided data and the theme's logical handling. Done means the commands list and related setup are no longer hard-coded in the theme and can be supplied independently by each consumer project.

Written by the indexing model from the issue text.