feat: Script for creating new theme plugin

[bmingles]

Added a script for creating new theme plugin. See README for instructions.

[mofojed]

Documenting what we discussed in our 1:1 here.
I want to take a look at using something like cookiecutter (doesn't necessarily have to be that package) for making a template. We publish a package that you can use to create a template, e.g. npx @deephaven/create-plugin, and then we can have templates for Theme plugins, or any other type of plugin.
Seems like the best option because it doesn't require cloning a repo to run a script.
Of course when we have python packaging up our JS plugins, we may want to use a more powerful templater (like yeoman or cookiecutter).
I just think we can do better than requiring someone clone our repo.

[bmingles]

Will be addressed by #256