improve: [line-numbers-mode] - Sync line format with lines that are displayed

[Encryptoid]

Description

When using a lower number format than the default(6), each line only displays the length that matches that line, as shown in the image:

It would be good to match most other editors by having the actual format sync with the lines that are being displayed, so all lines-numbers have the same width. This means when showing 2 digit lines, and you scroll down to a 3 digit line, the 2 digit lines will expand their width to match the 3 digits.

I imagine this is why a high default of 6 was chosen, but it produces a very large block like this:

It would be good to reduce the default after fixing.

Steps to reproduce

Set a lower format like this: (or open a file with 1M+ lines!)

(setf (lem:variable-value 'lem/line-numbers:line-number-format :global) "~1D ")

Recommendations

I am not sure if Lem has a concept yet of which lines are currently being displayed, but this might be a lot easier after that.

It also might not be directly in the handling of the string format, but I think you can also pass a function to work out the line number.