feat: offer quickfix to add spaces separating letters in unknown math var (#2062)

Examples:

| Input | Suggested fix |
| -- | -- |
| `$xyz$` | `$x y z$` |
| `$a_ij$` | `$a_(i j)$` |
| `$a_(ij)$` | `$a_(i j)$` |
| `$xy/z$` | `$(x y)/z$` |

If the unknown identifier appears as a subscript or as the
numerator/denominator of a fraction, we parenthesize the suggested fix.
For example, `a_ij` turns into `a_(i j)`, not `a_i j`, because the
latter is unlikely to be what the user intended.

crates/tinymist-query/src/diagnostics.rs

@@ -46,8 +46,16 @@ impl<'w> DiagWorker<'w> {
         }
     }
 
-    /// Runs code check on the document.
-    pub fn check(mut self) -> Self {
+    /// Runs code check on the given document.
+    pub fn check(mut self, source: &Source) -> Self {
+        for diag in self.ctx.lint(&source) {
+            self.handle(&diag);
+        }
+        self
+    }
+
+    /// Runs code check on the main document and all its dependencies.
+    pub fn full_check(mut self) -> Self {
         for dep in self.ctx.world.depended_files() {
             if WorkspaceResolver::is_package_file(dep) {
                 continue;