DISCNUMBER is not read from FLAC files

[amaendle]

Describe the bug
When reading FLAC files with jsmediatags, the DISCNUMBER tag is not returned, even though it is present and readable by other taggers (itt is even shown on the file properties in Windows).

Expected behavior
DISCNUMBER should be available in the parsed tags object, similar to TRACKNUMBER.

Actual behavior
The tag is missing from the result when parsing FLAC files.

Steps to reproduce

1. Use a FLAC file that contains a DISCNUMBER tag (e.g. Vorbis comment DISCNUMBER=1). This can be done even in the Windows explorer (right click, -> properties)
2. Parse it with jsmediatags
3. Observe that DISCNUMBER is not present in the returned tags

Code example

jsmediatags.read(file, {
  onSuccess: ({ tags }) => {
    console.log(tags);
  },
  onError: console.error
});