[analysis_server] Add an LSP handler for editing argument values

This adds basic editing for the argument types supported by `textDocument/editableArguments`. It doesn't currently attempt to format code currently (or use multiline/raw strings where they might be more appropriate), and does not allow arbitrary values that don't match the type for the parameter.

Change-Id: Ied95d95c0de39310aef0a1153864e902d56e1910
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395280
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Elliott Brooks <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: DanTup

pkg/analysis_server/lib/src/lsp/constants.dart

@@ -160,6 +160,11 @@ abstract final class CustomMethods {
     'experimental/dart/textDocument/editableArguments',
   );
 
+  /// Method for adding/editing an argument at a location in a document.
+  static const dartTextDocumentEditArgument = Method(
+    'experimental/dart/textDocument/editArgument',
+  );
+
   // TODO(dantup): Remove custom AnalyzerStatus status method soon as no clients
   //  should be relying on it as we now support proper $/progress events.
   static const analyzerStatus = Method(r'$/analyzerStatus');

pkg/analysis_server/lib/src/lsp/handlers/custom/editable_arguments/editable_arguments_mixin.dart

@@ -21,11 +21,8 @@ typedef EditableInvocationInfo =
       int numSuppliedPositionals,
     });
 
-/// A mixin that provides functionality for locating arguments and associated
-/// parameters in a document to allow a client to provide editing capabilities.
 mixin EditableArgumentsMixin {
-  /// Gets information about an invocation at [offset] in [result] that can be
-  /// edited.
+  /// Gets the argument list at [offset] that can be edited.
   EditableInvocationInfo? getInvocationInfo(
     ResolvedUnitResult result,
     int offset,