Add GIF format to document

[hata6502]

Add GIF format to docs/image-format.md tests/constants.mjs according to the below:

• https://github.com/naptha/tesseract.js/blob/master/tests/constants.mjs#L17
• https://tesseract-ocr.github.io/tessdoc/InputFormats.html

I tested to recognize a GIF image!
https://tesseract.projectnaptha.com/
example.gif

[Balearica]

The reason I was hesitant to add GIF support to the documentation is that "GIF" has become largely synonymous with animated images. The term "GIF" is frequently used to describe animated images of other formats, and it's rare to encounter non-animated .gif files in the wild.

That being said, this is a supported format and the documentation should be complete. I edited your branch to clarify that non-animated .gif files are supported, and will merge.