Consider adding MIP number to the filename of a MIP

[t4top]

Currently, MIPs markdown files don't have their MIP numbers in their filenames. This make sorting difficult and will become harder to navigate as more MIPs are proposed. I suggest to have MIP numbers in the filenames just like Ethereum EIPs and Cardano CIPs repos.

A MIP filename can follow below naming convention:

mip-<number>-<feature-name>.md

Where <number> is the MIP number in a 4-digit format, left-padded with 0, and <feature-name> is hyphen-separated name of the title or major feature of the MIP.

For example, mip-0001-remove-supercharged-rewards.md instead of mip-remove-supercharged-rewards.md, or mip-0004-zkapps.md instead of mip-zkapps.md.