changeable nat64 prefix, and large-ish UX changes

[BPplays]

backend

what this adds:

• changeable nat64 prefix
• a proper comparison of subnets instead of just text
• optional dot-decimal notation for the v4 part of a nat64 address
• changeable ipv4 format

there is also no automatic nat64 prefix detection which is a feature some people seemed to want.
doing that in the extension itself seems like it would require pinging something like ipv4.google.com and,
you'd probably want caching,
there'd be a tradeoff between how often you check the nat64 prefix and how many network requests you make,
for something like a laptop or phone you might want to update every few hours but longer would be better for stationary devices

you could also try to use a separate program like Yomitan can (for a different purpose) to get the nat64 prefix from the OS.

UX

changes:

• changes onclick to onmousedown for any action the user wouldn't want to cancel
• changes the style for like everything.
• checks if the user has entered a valid ipv6 address before accepting it.
• changes options menu font to just sans-serif instead of defaulting to Noto Sans and Arial, im not sure the reason that was done i think using the users selected font works better here

known issues:

• firefox wants to drag the background when you hold mouse down, this is even true for good versions like floorp. but it seems to also happen to other extensions with similar options menus.
• the scrollbar seems to sometimes oscillate on and off for certain window sizes and delay times.

onmousedown is used is responsiveness for anything the user wouldn't want to cancel, like checking a checkbox or selection a radio button.
i've also tested on mobile and it works fine there.

also if you have any other ideas for onmousedown for checkboxes and radio buttons let me know, the current method is kinda janky, though i haven't had any issues with it so far.

misc

this changes the makefile make an unpacked chrome build, i didn't see any instructions for a development workflow so i just loaded the unpacked extension and then reloaded it when i changed something, i can remove it from the makefile if you want.