Improve tooltip consistency, clarity, and look.

[wcarss]

• prepend a 'v' to the version number headings to make it clearer what they represent, as someone not familiar with rust may not immediately infer that '1.42' on its own is a version number
• changed perfDelta to always return a number, rather than a formatted string, and put the string formatting in the tooltip's formatter function, a little bit more cleanly shown with string template
• also use toFixed for delta+value numbers rather than string manip to consistently round off to 2 decimals, where before there were odd artifacts like "(12.%)" sometimes
• changed the separator to remove the leading space, because a string like "key : value" is less common convention than "key: value".

before: after:

[nindalf]

Sorry about this, I think there's a merge conflict.

Changes look good otherwise!

[nindalf]

I see Cloudflare still won't comment. No matter, I'll merge this since there are no merge conflicts.

[wcarss]

it seems the tooltip line got included in the changes -- resolved :)

[nindalf]

Seems like the build failed.

[nindalf]

Build issue is now fixed after updating recharts.