vscode: make keybindings submodule freeform

vscode supports overrides per platform key combinations at `mac`, `win` and `linux`

Enables `freeformType = jsonFormat` in the keybindings submodule. This enables support for per-platform bindings and any other arbitrary keybinding attributes.

Code pointer: 83b7ce68a7/src/vs/workbench/services/keybinding/browser/keybindingService.ts (L105-L120)

modules/programs/vscode/mkVscodeModule.nix

@@ -170,6 +170,7 @@ let
         type = types.either types.path (
           types.listOf (
             types.submodule {
+              freeformType = jsonFormat.type;
               options = {
                 key = mkOption {
                   type = types.str;