Use getPossibleBreakpoints to support column bps, when supported

[roblourens]

https://chromedevtools.github.io/debugger-protocol-viewer/v8/Debugger/#method-getPossibleBreakpoints

I think when a BP is set at column x, need to call getPossibleBPs for line N, from column 0 to column x, and pick the last one.

If the runtime doesn't support that method, need to set the BP at column 0 (pre-sourcemapping)

[auchenberg]

Nicely spotted. This API must be used by Chrome's new inline-breakpoints feature.