feat(unstable/fmt): move yaml formatting behind unstable flag (#24848)

This moves YAML formatting behind an unstable flag for Deno 1.46. This
will make it opt-in to start and then we can remove the flag to make it
on by default in version of Deno after that.

This can be specified by doing `deno fmt --unstable-yaml` or by
specifying the following in a deno.json file:

```json
{
  "unstable": ["fmt-yaml"]
}
```

cli/args/mod.rs

@@ -279,9 +279,15 @@ impl BenchOptions {
   }
 }
 
+#[derive(Clone, Debug, Default, PartialEq, Eq, Hash)]
+pub struct UnstableFmtOptions {
+  pub yaml: bool,
+}
+
 #[derive(Clone, Debug)]
 pub struct FmtOptions {
   pub options: FmtOptionsConfig,
+  pub unstable: UnstableFmtOptions,
   pub files: FilePatterns,
 }
 
@@ -295,13 +301,21 @@ impl FmtOptions {
   pub fn new_with_base(base: PathBuf) -> Self {
     Self {
       options: FmtOptionsConfig::default(),
+      unstable: Default::default(),
       files: FilePatterns::new_with_base(base),
     }
   }
 
-  pub fn resolve(fmt_config: FmtConfig, fmt_flags: &FmtFlags) -> Self {
+  pub fn resolve(
+    fmt_config: FmtConfig,
+    unstable: UnstableFmtOptions,
+    fmt_flags: &FmtFlags,
+  ) -> Self {
     Self {
       options: resolve_fmt_options(fmt_flags, fmt_config.options),
+      unstable: UnstableFmtOptions {
+        yaml: unstable.yaml || fmt_flags.unstable_yaml,
+      },
       files: fmt_config.files,
     }
   }
@@ -1306,14 +1320,21 @@ impl CliOptions {
     let member_configs = self
       .workspace()
       .resolve_fmt_config_for_members(&cli_arg_patterns)?;
+    let unstable = self.resolve_config_unstable_fmt_options();
     let mut result = Vec::with_capacity(member_configs.len());
     for (ctx, config) in member_configs {
-      let options = FmtOptions::resolve(config, fmt_flags);
+      let options = FmtOptions::resolve(config, unstable.clone(), fmt_flags);
       result.push((ctx, options));
     }
     Ok(result)
   }
 
+  pub fn resolve_config_unstable_fmt_options(&self) -> UnstableFmtOptions {
+    UnstableFmtOptions {
+      yaml: self.workspace().has_unstable("fmt-yaml"),
+    }
+  }
+
   pub fn resolve_workspace_lint_options(
     &self,
     lint_flags: &LintFlags,
@@ -1640,8 +1661,12 @@ impl CliOptions {
           .map(|granular_flag| granular_flag.0)
           .collect();
 
-      let mut another_unstable_flags =
-        Vec::from(["sloppy-imports", "byonm", "bare-node-builtins"]);
+      let mut another_unstable_flags = Vec::from([
+        "sloppy-imports",
+        "byonm",
+        "bare-node-builtins",
+        "fmt-yaml",
+      ]);
       // add more unstable flags to the same vector holding granular flags
       all_valid_unstable_flags.append(&mut another_unstable_flags);