added use of ABC to mark TextGenerator as abstract

Proposed changes

Use Abstract Base Class (ABC) to mark text generator as abstract

Types of changes

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[] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Change to tutorials, tests or examples
[ ] Non code change (readme, images or other non-code assets)
[ ] Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

[x] Lint and unit tests pass locally with my changes
[x] I have added tests that prove my fix is effective or that my feature works
[ ] I have added necessary documentation (if appropriate)
[ ] Any dependent changes have been merged and published in downstream modules
[ ] Submission does not reduce code coverage numbers
[ ] Submission does not increase alerts or messages from prospector / lint

Further comments

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databrickslabs / dbldatagen