feat: add range formatting to formatter

[guru245]

Range formatting in Conform is a standout feature that many other formatters lack. However, it was previously missing from kickstart. I’ve now added it, based on the guidance from this section of the conform.nvim documentation.

This change is fully backward-compatible. Existing users can continue to use <leader>k to format the entire file as before. If they select a block of code and press <leader>k, only the selected range will be formatted.