Provide diagnostic marker API in $psEditor

[daviwil]

It would be useful if there was an API to add diagnostic markers to script files so that any editor command could add "squigglies" to arbitrary regions of an open file. Markers would be added in relation to a named "layer" which would allow Editor Services to keep track of a unique set of markers for each module or use case.

The API might look something like this:

$file = $context.CurrentFile
# OR: $file = $psEditor.Workspace.GetFile("filePath.ext")
$file.SetMarkers("MarkerLayerName", markerArray)

We should also provide an API for clearing markers, either for a specific layer or for all layers:

$file.ClearMarkers("MarkerLayerName")
$file.ClearMarkers()

Once this API exists, I'll use it to implement the new integration model for PSScriptAnalyzer in issue #213.

FEEDBACK NEEDED: Is SetMarkers good enough or should it be SetFileMarkers, SetDiagnosticMarkers or something else?