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"]
}
```

2 files changed, 28 insertions, 0 deletions

diff --git a/tests/specs/fmt/unstable_yaml/__test__.jsonc b/tests/specs/fmt/unstable_yaml/__test__.jsonc
new file mode 100644
index 000000000..885db59b9
--- /dev/null
+++ b/tests/specs/fmt/unstable_yaml/__test__.jsonc
@@ -0,0 +1,25 @@
+{
+  "tempDir": true,
+  "tests": {
+    "nothing": {
+      "args": "fmt",
+      "output": "Checked 1 file\n"
+    },
+    "flag": {
+      "args": "fmt --unstable-yaml",
+      "output": "[WILDLINE]badly_formatted.yml\nChecked 1 file\n"
+    },
+    "config_file": {
+      "steps": [{
+        "args": [
+          "eval",
+          "Deno.writeTextFile('deno.json', '{\\n  \"unstable\": [\"fmt-yaml\"]\\n}\\n')"
+        ],
+        "output": "[WILDCARD]"
+      }, {
+        "args": "fmt",
+        "output": "[WILDLINE]badly_formatted.yml\nChecked 2 files\n"
+      }]
+    }
+  }
+}
\ No newline at end of file
diff --git a/tests/specs/fmt/unstable_yaml/badly_formatted.yml b/tests/specs/fmt/unstable_yaml/badly_formatted.yml
new file mode 100644
index 000000000..49646f320
--- /dev/null
+++ b/tests/specs/fmt/unstable_yaml/badly_formatted.yml
@@ -0,0 +1,3 @@
+- Test
+ - Test
+  - Test
\ No newline at end of file