New patch attached.

Faces are now 'line-number-Nth-line' and 'line-number-Mth-line', and customizable options to set the numbers are 'display-line-numbers-Nth-line' and 'display-line-numbers-Mth-line'. Better names, or wording, or customization type, are welcome.

Both options default to 0 and so are initially disabled. (But I've set up the customization so when setting them, 10 and 5 are suggested as default values.)