Update deps, a few majors

[jakebailey]

Updates dependencies; no more npm audit messages.

• Updates compatible versions
• Switches to picocolors, already used by other deps (and now has the light helpers which means it is a drop in for chalk, which we had been holding back)
• Update to the latest version of diff, which has breaking changes but none that affect us.
• Update to chokidar 4, which theoretically would reduce the number of deps, but mocha still depends on v3 (and even the latest mocha does too).
  • v4 doesn't do globs, but we don't need them anyway, only a recursive watch on the test cases.

To support older versions of node, we're still holding back chai, glob, mocha, ms, which.