chore: export type MailDataRequired

[antonsamper]

Exposes the new mail properties type MailDataRequired that was added in #1041

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
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• I have added tests that prove my fix is effective or that my feature works
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• I have added inline documentation to the code I modified

[childish-sambino]

@antonsamper Thoughts on test failure?

packages/mail/src/mail.d.ts:36:1 - error TS2309: An export assignment cannot be used in a module with other exported elements.

[antonsamper]

@childish-sambino looking into it now.
I'm going to move the type declaration to @sendgrid/helpers/classes/mail along with the others and will export it from there.

[childish-sambino]

@antonsamper Make sense to me. Can you merge in this change as well? #1054

[FelixLeChat]

Is there an ETA for a release containing this fix ?

[childish-sambino]

Next release is scheduled for 2020-03-18.

[rubenheymans]

What is the temp fix though?

[childish-sambino]

@rubenheymans Temp fix is to pin to the 6.5.3 release.