Clean up and Document the io.image enhancements

[datumbox]

PR #3024 introduced a few enhancements on the io.image methods used for reading JPEG and PNG images. The code changes were merged but there are a few nice-to-have improvements missing.

More specifically:

• Provide better error messages for JPEG and PNG when a specific ImageReadMode value is not supported.
• Ensure that image_read_mode.h has a new line at the end of the file.
• Replace the old #define with the newer constexpr expression at image_read_mode.h.
• Move the documentation of ImageReadMode values from the methods to the enum.

Note: constexpr might cause problems on Windows. If that's the case, try using const instead.

[Sid2697]

Hey! I was wondering if I could contribute to this issue, is there anyone already working on it?

[datumbox]

Hey @Sid2697, welcome!

Go for it. Looking forward to your PR.