Add assert_rbi_exists? to support wildcard

[KaanOzkan]

Using project assertions with RBI's is common in Tapioca and allowing assert_rbi_exists?("sorbet/rbi/gems/foo@*.rbi") would be beneficial so that we can omit the version number.

Maybe we can first check if the given string is a file. If not we can also search the directory with the given path which will support the wildcard out of the box Dir["sorbet/rbi/gems/foo@*.rbi"]

I thought this is mostly for RBIs so I'm suggesting a new method but we could also modify the existing assert_project_file_exist.
Related PR where we had to update an assertion Shopify/tapioca#2046

[Morriar]

This may be better suited for https://github.com/Shopify/tapioca/blob/main/spec/spec_with_project.rb#L87? This is where we define all the custom assertions used in Tapioca test suite.