Support dedicated lane for breakpoint decorations

[joyceerhl]

For #179725

This PR adds support for a dedicated breakpoint decoration lane, so that you can always see both breakpoints and at least one other decoration. The behavior implemented is as follows:

• When there are no decorations of any sort in an editor, one lane is rendered
• When there are only breakpoints, one lane is rendered
• When there are only non-breakpoint decorations, one lane is rendered and the zIndex for the decoration is used to determine which one is rendered (until we have overflow implemented)
• When there are both breakpoints and decorations but they do not appear on the same line, one lane is rendered
• When there are both breakpoints and decorations and they appear at least once on the same line, two lanes are rendered

dedicated-debug-lane.mp4

Notes:

1. I suspect we actually want to have two lanes if there are only non-breakpoint decorations and breakpoints can be set, in order to avoid resizing the editor gutter and breaking muscle memory for rendering the breakpoint hint on a line that already has a non-breakpoint decoration. However, this will involve some more changes to the breakpointEditorContribution.ts, which currently doesn't render the breakpoint hint on hover if there are any other decorations on the same line, so I figured I'd implement that in a separate PR.
2. I have implemented the breakpoint to always appear in the rightmost lane for now (rationale being that it's closer to the line number). Let me know if you think it should actually appear in the leftmost lane.
3. I've left open the possibility of showing more than two lanes, in case users want the margin to resize to accommodate unlimited decorations. However I don't intend to expose a setting for this until we receive further feedback to that effect.

[alexdima]

Nice! 👏