Allow comma-separated patterns for folder filters when merging

[adam-coster]

It's more likely that someone will want to list a set of folders, than that they'd want to provide a regex, when filtering on merge.

For example, if a user wanted to import two modules, "TestSuite" and "MyRadModule", they'd currently have to use:

stitch merge --if-folder-matches "(TestSuite|MyRadModule)"

Since most people are not very familiar with regex, it would make more sense to have it not behave has a regex pattern by default, and to also allow comma-separated values. For example:

stitch merge --if-folder-matches TestSuite,MyRadModule

If the user wants to use a regex, we can either:

• Switch modes when a non-varname character appears (e.g. if []()$^*+ appear in the filter string).
• Have a separate option flag for regex-based matching.