[FIX] web: btn-group, allow gap control

Before this commit, due to the milk design, there was a gap between the
buttons in btn-group. However, in certain modules, it was necessary to
have a negative margin-left to achieve an edge-to-edge appearance.

To address this issue, I introduced a CSS variable called
--btn-group-gap to control the margin-left property for the elements.
This change allows for customization of the margin-left value, with a
fallback value of $btn-border-width by default, when needed.

task-3336458
part of task-3326263

closes odoo/odoo#126068

X-original-commit: 1a82e8e80745d26b8c96cd5bf0c10f14b793d440
Related: odoo/enterprise#43001
Signed-off-by: Pierre Paridans (app) <app@odoo.com>

addons/web/static/src/scss/bootstrap_review_backend.scss

@@ -91,7 +91,7 @@
         }
 
         &:not(:first-child) {
-            margin-left: $btn-border-width;
+            margin-left: var(--btn-group-gap, $btn-border-width); 
         }
     }
 }