Pull childless @layer rules to the top of the output stylesheet

These rules are only allowed at the beginning of stylesheets. Allowing
them anywhere in Sass and moving them to the top of the output matches
the behavior of plain-CSS `@import` rules.

Closes #3842

spec/at-rules/extend.md

@@ -86,6 +86,11 @@ shorthands:
 * `extend(extendee, extensions)` for iteratively running `extendee =
   extend(extendee, extension)` for each `extension` in `extensions`.
 
+### Initial Rule
+
+An "initial rule" is a CSS at-rule that is either an `@import` or a `@layer`
+rule with no children.
+
 ## Semantics
 
 The `@extend` rule means that all elements matching the [extender](#extender)
@@ -208,26 +213,44 @@ that includes CSS for *all* modules transitively used or forwarded by
     > Because this traverses modules depth-first, it emits CSS in reverse
     > topological order.
 
-  * Let `initial-imports` be the longest initial subsequence of top-level
-    statements in `domestic`'s CSS tree that contains only comments and
-    `@import` rules *and* that ends with an `@import` rule.
+  * Let `initial` be the longest initial subsequence of top-level statements in
+    `domestic`'s CSS tree that contains only comments and [initial rules], *and*
+    that ends with an initial rule.
+
+  * Otherwise, if `initial` contains a `@layer` rule `layer`:
+
+    * If `css` already contains a `@layer` rule without children, throw an
+      error.
+
+    * Otherwise, remove `layer` from `initial` and insert a copy of it at the
+      beginning of `css` after any comments.
 
-  * Insert a copy of `initial-imports` in `css` after the last `@import` rule, or
-    at the beginning of `css` if it doesn't contain any `@import` rules.
+  * Insert a copy of `initial` in `css` after the last initial rule, or at the
+    beginning of `css` after any comments if it doesn't contain any initial
+    rules.
 
   * For each top-level statement `statement` in `domestic`'s CSS tree after
-    `initial-imports`:
+    `initial`:
 
-    * If `statement` is an `@import` rule, insert a copy of `statement` in `css`
-      after the last `@import` rule, or at the beginning of `css` if it doesn't
-      contain any `@import` rules.
+    * If `statement` is a `@layer` rule without children:
+
+      * If `css` already contains a `@layer` rule without children, throw an
+        error.
+
+      * Otherwise, insert a copy of `statement` at the beginning of `css`.
+
+    * Otherwise, If `statement` is an `@import` rule, insert a copy of
+      `statement` in `css` after the last initial rule, or at the beginning of
+      `css` if it doesn't contain any initial rules.
 
     * Otherwise, add a copy of `statement` to the end of `css`, with any style
       rules' selectors replaced with the corresponding selectors in
       `new-selectors`.
 
 * Return `css`.
 
+[initial rules]: #initial-rule
+
 ### Extending a Selector
 
 This algorithm takes a selector list `extendee`, a simple selector `target`, and