Put breakpoints on the line number margin and let a file type restrict them

[danielpourbakhsh]

Three changes around the editor margin.

Breakpoints move onto the line numbers. A line that carries one shows the dot in place of its number, as Rider and VS Code do. MeasureOverride stays inherited, so the column is exactly as wide as it was before.

BreakPointMargin stays and is marked [Obsolete] — nothing in the tree points at it any more, but it is public API. It loses a leak on the way: the store outlives the margin, so the subscription moves into OnTextViewChanged and is released when the margin detaches.

One store instead of one per editor.ExtendedTextEditor handed every margin a new BreakpointStore(), so a breakpoint lived in an object that no debug session ever saw. The margin gets BreakpointStore.Instance now, and a breakpoint survives closing and reopening its file.

A file type can restrict which lines take a breakpoint.ITypeAssistance.BreakPointLinePattern is a regular expression, null meaning no restriction — a language without a rule notices nothing of this. Assembly needs it: a breakpoint on a data word does not fail, it slips, because the line table has no entry for it and the backend moves it to the next line that has code. The rule travels as data and not as a predicate, so the core never calls into plugin code on a click or a pointer move. Removing a breakpoint always stays possible, and the preview dot appears only where a click would take effect.

SetEnableBreakpoints takes the ITypeAssistance instead of a bool, because the margin needs the flag and the pattern as a unit.

Also in here: BreakPoint records whether the target armed it and the margin draws a grey hollow ring when it did not; the store raises VerificationChanged for that, since the change alters no collection and a margin listening only to CollectionChanged would never repaint. The store also says whether the target is running — while it is, the margin takes no breakpoint in either direction and says why. Both stay inert until a debug session sets them.

Builds on its own against main.