[FIX] web_editor: resolved NaN issue in font-size dropdown in toolbar

Commit [1] resolved an issue related to the behavior of small font sizes, which
caused 'NaN' to appear in the font-size dropdown within the floating toolbar.
This occurred due to the removal of a variable definition, resulting in the
inability to compute the font size. This commit rectifies the problem by
reintroducing the variable in the SCSS file to ensure correct rendering.

[1]: 7931d1a14a

task-3801894

closes odoo/odoo#158811

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/scss/web_editor.common.scss

@@ -90,6 +90,7 @@
     @include print-variable('h5-font-size', $h5-font-size);
     @include print-variable('h6-font-size', $h6-font-size);
     @include print-variable('font-size-base', $font-size-base);
+    @include print-variable('small-font-size', $small-font-size);
 }
 
 html, body {
@@ -292,7 +293,7 @@ img.ms-auto, img.mx-auto {
 // "o_small" class currently also changes the font-size only but this is to stay
 // consistent with the other classes which act that way (as display-x).
 .o_small-fs {
-    @extend %o-small-font-size;
+    @include font-size($small-font-size);
 }
 
 div.media_iframe_video {