FITS: Add missing `application/fits` and legacy globs

Make image/fits an alias, matching its designation of a subset of the FITS standard, as per https://datatracker.ietf.org/doc/html/rfc4047#section-5 -

"The general nature of the full FITS standard requires the use of the media type "application/fits". Nevertheless, the principal intent for a great many FITS files is to convey a single data array in the PHDU, and such arrays are very often 2-dimensional images. Several common image viewing applications already display single-HDU FITS files, and the prototypes for virtual observatory projects specify that data provided by web services be conveyed by the data array in the PHDU. These uses justify the registration of a second media type, namely "image/fits", for files which use the subset of the standard described by the original FITS standard paper."

The existing alias image/x-fits was common before the standardization of application/fits and image/fits, so it should stay.

Current standards and historical contexts can be found here:

• https://fits.gsfc.nasa.gov/fits_standard.html
• https://listmgr.nrao.edu/pipermail/fitsmime/2002-December/thread.html
• https://www.ucolick.org/~sla/fits/mime/comments.html

Add missing eight-dot-three-style *.fit and *.fts globs for legacy files, matching the list supported by libvips, i.e., https://github.com/libvips/libvips/blob/757d03100566368e5d06c7355e73c1215dd34968/libvips/foreign/fits.c#L132

And as discussed in the FITS standard definition (fits_standard40aa-le.pdf) -

"In the absence of other information it is reasonably safe to presume that a file name ending in ‘.fits’ is intended to be a FITS file. Nevertheless, there are other commonly used ex- tensions; e.g., ‘.fit’, ‘.fts’, and many others not suitable for listing in a media type registration."