fix: update MailService constructor property definition in TypeScript declaration

[dhritzkiv]

Before, {MailService: MailService} suggested an instance of MailService was being passed. Changing the syntax to {new(): MailService} typeof MailService suggests that it's a callable constructor reference available as a property

Fixes

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the development branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

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[SendGridDX]

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[thinkingserious]

@spartan563,

Do you mind giving this one a quick look? Thanks!

[notheotherben]

Good catch, however I believe you can also use typeof MailService to more correctly reflect the fact that the property is actually exposing the full (uninstantiated) type.

[dhritzkiv]

@spartan563 you're right; made the change to typeof

[notheotherben]

Looks great, thanks Daniel.

[bs85]

@thinkingserious any reason this wasn't merged? This is forcing us to use ugly hacks anywhere we need to create a new instance of that class.

[thinkingserious]

Hello @dhritzkiv,

Thanks again for the PR!

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