std.time: renamed Timer to Stopwatch

[zacoons]

"Timer" is commonly associated with something which triggers after a specified amount of time has elapsed.
"Stopwatch" is closer to what this class is for: measuring how much time has elapsed since it began.

This will be a breaking change for many.

@nektro suggested I open a PR to get comments from the dev team.

[xydone]

The only mainstream programming language (which I'm aware about) which implements something that behaves the same way as Zig's std.time.Timer and calls it Stopwatch is C#, most other languages have neither of the two in the name and opt for different, albeit also ambiguous wording. While Stopwatch is the more semantically more accurate than Timer, I think this is just a breaking change with no massive benefits to it.