Add support for breakpoints in debugger

Breakpoints should persist for the session.

breakpoint set <t>]

Set a breakpoint a given time

Example: breakpoint set 4.0
breakpoint list

List all active breakpoints.
breakpoint delete [<id> | --all]

Delete the breakpoint with the specified id.
Providing the --all option removes all breakpoints.

Example: breakpoint delete 1

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