Test extension API for breakpoints

[weinand]

Test for #23188:

Complexity: 4

• Any OS - @jrieken

The November milestone of VS Code proposes extension API for reading the breakpoints of a workspace and tracking added, removed, and changed breakpoints:
https://github.com/Microsoft/vscode/blob/a42cd0efc5b4baa17075fcd8da1c5e2097419c6f/src/vs/vscode.proposed.d.ts#L251-L329

Verify:

• API makes sense (especially the Breakpoint, SourceBreakpoint, FunctionBreakpoint hierarchy and its use of the type discriminator). Will this work if we extend the API to create those types?
• write a simple extension that accesses breakpoints and registers for BreakpointsChangeEvents. Please note that accessing breakpoints initially returns an empty array but triggers a subsequent event that has the full set of breakpoints in its added property.