Update package name

[TomRHandcock]

• Updates package name to mirage_generator.
• Removes 'How do I install it?' section from README in preparation for pub.dev publication.
• Bump package version.

[SamC-Apadmi]

Happy to approve this but I did want to suggest maybe chancing the name completely to avoid confusion from the other mirage?

I'm just thinking about packages like riverpod that also have a generator? Before we go to pub.dev I guess is the last opportunity if we want to do that?

Will follow your lead though @TomHa-Apadmi

[TomHa-Apadmi]

@SamC-Apadmi Yeah you make a good point we're locked in once we do the first publish. I'll have a brainstorm over the weekend about other package names. Problem is I've fallen into being fairly married to 'mirage' 😆