Added webp preset types and test for same. Issue 3747

[pilotso11]

For issue #3747.

The webp output options type is missing the preset values in the types. They are present in the js library.
This PR adds:

• A new enum (PresetEnum)
• The preset on WebpOptions
• The enum as sharp.preset on sharp.
• Tests for both string and enum based use of webp({preset: ...})

This follows the same implementation as other options, such as resize.fit.

npm test-types and npm test-lint both run cleanly.

[lovell]

Thank you very much for the PR, I've left one question inline.

[lovell]

The enum as sharp.preset on sharp.

Did you intend to include this as part of your PR?

[pilotso11]

| The enum as sharp.preset on sharp.
I had added this to be consistent with the way other enum's seem to have been exposed. It does allow the following two code styles:

sharp('input.tiff').webp({ preset: 'photo' }).toFile('out.webp');
sharp('input.tiff').webp({ preset: sharp.webpPreset.photo }).toFile('out.webp');

But having taken a closer look, the other exposed enums are there specifically because they are then exposed and overridden in an interface later. So this doesn't really seem necessary. I've removed it and pushed the code again.

[lovell]

Thank you Seth!