[web_generator] Support for Single IDL File Parsing

[nikeokoronkwo]

This issue is for supporting the parsing of individual IDL files, rather than the default of the ones provided for package:web.
This will most likely involve adding support on dart_main.dart for individual IDL files, and default to normal behaviour if an input file isn't provided.

This can help us run integration tests on the generation process for IDL files.