fix(types): Exports SpecificErrorClass from main.d.ts

[benkroeger]

Fixes #35

Which problem is this pull request solving?

TypeScript error TS2742 when calling subclass with plugins option

List other issues or pull requests related to this problem

This fixes #35

Describe the solution you've chosen

Example: I've fixed this by exporting SpecificErrorClass from main.d.ts

Describe alternatives you've considered

none

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[ehmicky]

Hi @benkroeger,

Thanks a lot for this PR!

This looks good. However, could you please add the following, as suggested from this previous PR #27:

• Put the export with the other ones at the bottom of src/main.d.ts
• Add the @private tag and the small comment, like in Fix for TS2742 (workaround) #27

Thanks!