Add sass entry file for frontend-shared

- Move svg-icon into frontend-shared/styles
- Update annotator.scss and sidebar.scss to reference new frontend-shared sass entry file

frontend-shared/styles/_index.scss

+// To use frontend-shared styles, include this file in the sass entry file(s) for parent app
+//
+// e.g.
+// @use '@hypothesis/frontend-shared/styles'
+
+@use './components/svg-icon';

src/styles/annotator/annotator.scss

@@ -7,8 +7,9 @@
 @use '../mixins/molecules';
 @use '../mixins/reset';
 
-// Shared components.
-@use '../components/svg-icon';
+// Shared styles
+// -----------------
+@use '@hypothesis/frontend-shared/styles';
 
 // Annotator-specific components.
 @use './components/adder';

src/styles/sidebar/sidebar.scss

@@ -7,9 +7,9 @@
 @use '../util';
 @use './elements';
 
-// Shared components
+// Shared styles
 // -----------------
-@use '../components/svg-icon';
+@use '@hypothesis/frontend-shared/styles';
 
 // Components
 // ----------