utils/vscode: Change assembly file ext to .spvasm (#2995)

Instead of `.spirv`.
`.spvasm` is the official extension for these textual files.
diff --git a/utils/vscode/package.json b/utils/vscode/package.json
index ccca687..d9c74e8 100644
--- a/utils/vscode/package.json
+++ b/utils/vscode/package.json
@@ -18,7 +18,7 @@
                 "id": "spirv",

                 "configuration": "spirv.configuration.json",

                 "extensions": [

-                    "spirv"

+                    "spvasm"

                 ]

             }

         ],