Allow SCSS syntax blocks in the indented syntax

As part of the Indented syntax improvements proposal, a SCSS-in-Sass format was proposed in #3973. This was removed in #4002 based on feedback in #3991.

This syntax would allow authors to opt in to SCSS format for part of the document. A few of the use cases that were mentioned-

1. A terser option for short styles, for instance a {color: red}.
2. An inline alternative for authors who import .scss files that contain long values, for instance grid template or font declarations.
3. Useful for migrating existing stylesheets to the SCSS format.