New stack trace format

[marvinhagemeister]

Encountered this stack trace format in the wild, which we don't properly parse yet:

 spy() at Object.<anonymous> (/projects/preact/test-utils/test/shared/act.test.js:50:20 <- test-utils/test/shared/act.test.js:1245:18)

    warn('getting vnode.attributes is deprecated, use vnode.props') => 1 at Object.get (/projects/preact/debug/src/debug.js:271:13 <- debug/test/browser/debug.test.js:2052:21)