Auto merge of #13359 - feniljain:feat-must-use-option, r=Veykril

feat: add config for inserting must_use in `generate_enum_as_method`

Should fix #13312

Didn't add a test because I was not sure on how to add test for a specific configuration option, tried to look for the usages for other `AssistConfig` variants but couldn't find any in `tests`. If there is a way to test this, do point me towards it.

I tried to extract the formatting string as a common `template_string` and only have if-else for that, but it didn't compile :(

Also it seems these tests are failing:

```
test config::tests::generate_config_documentation ... FAILED
test config::tests::generate_package_json_config ... FAILED
```

Can you also point me to how to correct these 😅  ( I guess there is some command to automatically generate these? )

editors/code/package.json

@@ -397,6 +397,11 @@
                     "type": "boolean"
                 },
                 "$generated-start": {},
+                "rust-analyzer.assist.emitMustUse": {
+                    "markdownDescription": "Whether to insert #[must_use] when generating `as_` methods\nfor enum variants.",
+                    "default": false,
+                    "type": "boolean"
+                },
                 "rust-analyzer.assist.expressionFillDefault": {
                     "markdownDescription": "Placeholder expression to use for missing expressions in assists.",
                     "default": "todo",