Fix malformed typescript exports

[programminghoch10]

When using your module as a ES module like this

import telegramifyMarkdown from 'telegramify-markdown'

the following compile error occurs:

node_modules/telegramify-markdown/types/index.d.ts(12,3): 
error TS2666: 
Exports and export assignments are not permitted in module augmentations.

The issue is explained here
and it basically says you can't use export = and export together in one module
because it mixes up commonjs module and ES module syntax.

I found the simple fix is removing the export on UnsupportedTagsStrategy,
because the module will simply continue to work as is.
The autocompletion on the second parameter of convert also continues to work.

While at it you can add to your README somewhere that

import telegramifyMarkdown from 'telegramify-markdown'

is the ES module compatible import.

[skoropadas]

Thanks a lot for the PR!

[github-actions]

🎉 This PR is included in version 1.2.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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