Add .gitignore to templates if needed

[jodeleeuw]

We should add a default .gitignore to the directories created by these tools. It should include at least node_modules and dist. However, if the package is being added to jspsych-contrib or jspsych-timelines then there's already a gitignore file in the root directory that handles it. Maybe there is no harm in having the extra one, or maybe we should check before adding.

[cherriechang]

I think this is a good idea! I did some brief googling and I don't think nested .gitignore files are bad practice.

[cherriechang]

added in this commit

[jodeleeuw]

should we include node_modules too?