vscode-extensions: make pname optional

Many code in the wild still use `vscode-utils.buildVscodeExtension`
with the `name` argument, so it would be better to make it optional.

For example, a github code search of `buildVscodeExtension` returns 100+
results, most of them uses the `name` argument:

  https://github.com/search?q=buildVscodeExtension+language%3ANix&type=code&l=Nix

pkgs/applications/editors/vscode/extensions/vscode-utils.nix

@@ -11,7 +11,7 @@
 let
   buildVscodeExtension =
     a@{
-      pname,
+      pname ? null, # Only optional for backward compatibility.
       src,
       # Same as "Unique Identifier" on the extension's web page.
       # For the moment, only serve as unique extension dir.
@@ -37,9 +37,10 @@ let
         "vscodeExtUniqueId"
         "pname"
       ])
-      // {
-
+      // (lib.optionalAttrs (pname != null) {
         pname = "vscode-extension-${pname}";
+      })
+      // {
 
         passthru = passthru // {
           inherit vscodeExtPublisher vscodeExtName vscodeExtUniqueId;