Don't clear all breakpoints on every event

We get breakpoint set/remove events independently for each breakpoint,
yet we're clearing all of them each time. This results in the breakpiont
IDs within the PowerShell session increasing quickly, and isn't
necessary. This probably isn't the whole fix.

Author: andyleejordan

src/PowerShellEditorServices/Services/DebugAdapter/DebugService.cs

@@ -135,7 +135,7 @@ public DebugService(
         public async Task<BreakpointDetails[]> SetLineBreakpointsAsync(
             ScriptFile scriptFile,
             BreakpointDetails[] breakpoints,
-            bool clearExisting = true)
+            bool clearExisting = false)
         {
             var dscBreakpoints =
                 this.powerShellContext