Improve pub.dev score

[k5924]

Problem: pub.dev score is 110/130

Solution:

• Changed type enum to Type
• Added some brief documentation to most methods in email_validator

Note: Solves #41 For methods I didn't quite understand, I added a TODO comment

[fredeil]

Can you try running this locally? https://pub.dev/help/scoring#calculating-pub-points-prior-to-publishing

It checks the score

[k5924]

I ran it locally and I think it now has a full score: