Add space to completion trigger characters

Adding space to completion triggers lets us automatically give
completion results for parameter values. This also required building in
some support for marking completion results as "Incomplete". This means
that the client will not cache results when standard identifier
characters are typed.

Mainly, this solves the scenario where you type space, get file
completion results, and then type `-`. You expect to then get parameter
names, but without marking "Incomplete", it will instead filter the file
results.

Author: SeeminglyScience

src/PowerShellEditorServices/Services/TextDocument/Handlers/CompletionHandler.cs

@@ -46,7 +46,7 @@ public PsesCompletionHandler(
             // TODO: What do we do with the arguments?
             DocumentSelector = LspUtils.PowerShellDocumentSelector,
             ResolveProvider = true,
-            TriggerCharacters = new[] { ".", "-", ":", "\\", "$" }
+            TriggerCharacters = new[] { ".", "-", ":", "\\", "$", " " }
         };
 
         public override async Task<CompletionList> Handle(CompletionParams request, CancellationToken cancellationToken)
@@ -55,13 +55,16 @@ public override async Task<CompletionList> Handle(CompletionParams request, Canc
             int cursorColumn = request.Position.Character + 1;
 
             ScriptFile scriptFile = _workspaceService.GetFile(request.TextDocument.Uri);
-            IEnumerable<CompletionItem> completionResults = await GetCompletionsInFileAsync(
+            (bool isIncomplete, IReadOnlyList<CompletionItem> completionResults) = await GetCompletionsInFileAsync(
                 scriptFile,
                 cursorLine,
                 cursorColumn,
                 cancellationToken).ConfigureAwait(false);
 
-            return new CompletionList(completionResults);
+            // Treat completions trigged by space as incomplete so that `gci `
+            // and then typing `-` doesn't just filter the list of parameter values
+            // (typically files) returned by the space completion
+            return new CompletionList(completionResults, isIncomplete || request.Context.TriggerCharacter is " ");
         }
 
         // Handler for "completionItem/resolve". In VSCode this is fired when a completion item is highlighted in the completion list.
@@ -113,7 +116,7 @@ public override async Task<CompletionItem> Handle(CompletionItem request, Cancel
         /// <returns>
         /// A CommandCompletion instance completions for the identified statement.
         /// </returns>
-        internal async Task<IEnumerable<CompletionItem>> GetCompletionsInFileAsync(
+        internal async Task<(bool isIncomplete, IReadOnlyList<CompletionItem> matches)> GetCompletionsInFileAsync(
             ScriptFile scriptFile,
             int lineNumber,
             int columnNumber,
@@ -131,21 +134,32 @@ internal async Task<IEnumerable<CompletionItem>> GetCompletionsInFileAsync(
 
             if (result.CompletionMatches.Count == 0)
             {
-                return Array.Empty<CompletionItem>();
+                return (true, Array.Empty<CompletionItem>());
             }
 
             BufferRange replacedRange = scriptFile.GetRangeBetweenOffsets(
                 result.ReplacementIndex,
                 result.ReplacementIndex + result.ReplacementLength);
 
+            bool isIncomplete = false;
             // Create OmniSharp CompletionItems from PowerShell CompletionResults. We use a for loop
             // because the index is used for sorting.
             CompletionItem[] completionItems = new CompletionItem[result.CompletionMatches.Count];
             for (int i = 0; i < result.CompletionMatches.Count; i++)
             {
+                CompletionResult completionMatch = result.CompletionMatches[i];
+
+                // If a completion result is a variable scope like `$script:` we want to
+                // mark as incomplete so on typing `:` completion changes.
+                if (completionMatch.ResultType is CompletionResultType.Variable
+                    && completionMatch.CompletionText.EndsWith(":"))
+                {
+                    isIncomplete = true;
+                }
+
                 completionItems[i] = CreateCompletionItem(result.CompletionMatches[i], replacedRange, i + 1);
             }
-            return completionItems;
+            return (isIncomplete, completionItems);
         }
 
         internal static CompletionItem CreateCompletionItem(