[FIX] web_editor: bs4 '.is-invalid' not applied in technical modal

Adding bs4 `.is-invalid` class is supposed to add red borders on the
`form-control` element.

But the mixin `o-w-preserve-forms` prevents that style to be applied as it adds
`border: 1px solid $o-we-color-text-light;` on `.form-control`.
As the mixin selector has more strength (specificity), the bs4 is simply not
applied.

This is the case for `form-control` inside `o_technical_modal`.

closes odoo/odoo#27691

addons/web_editor/static/src/scss/web_editor.variables.scss

@@ -112,6 +112,9 @@ $o-we-switch-inactive-color: $o-we-color-paper !default;
         color: #555;
         background-color: #fff;
         border-radius: 0;
+        &.is-invalid {
+            border-color: $danger;
+        }
     }
 
     .was-validated {