Snippets: Move snippets into the core of editor (#13937)

Release Notes:

- Move snippet support into core editor experience, marking the official
extension as deprecated. Snippets now show up in any buffer (including
plain text buffers).

crates/snippet_provider/src/format.rs

@@ -0,0 +1,44 @@
+use collections::HashMap;
+use serde::Deserialize;
+
+#[derive(Deserialize)]
+pub(crate) struct VSSnippetsFile {
+    #[serde(flatten)]
+    pub(crate) snippets: HashMap<String, VSCodeSnippet>,
+}
+
+#[derive(Deserialize)]
+#[serde(untagged)]
+pub(crate) enum ListOrDirect {
+    Single(String),
+    List(Vec<String>),
+}
+
+impl From<ListOrDirect> for Vec<String> {
+    fn from(list: ListOrDirect) -> Self {
+        match list {
+            ListOrDirect::Single(entry) => vec![entry],
+            ListOrDirect::List(entries) => entries,
+        }
+    }
+}
+
+impl std::fmt::Display for ListOrDirect {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(
+            f,
+            "{}",
+            match self {
+                Self::Single(v) => v.to_owned(),
+                Self::List(v) => v.join("\n"),
+            }
+        )
+    }
+}
+
+#[derive(Deserialize)]
+pub(crate) struct VSCodeSnippet {
+    pub(crate) prefix: Option<ListOrDirect>,
+    pub(crate) body: ListOrDirect,
+    pub(crate) description: Option<ListOrDirect>,
+}