Add CSS variable for markdown-html overflow-x

Allow host components and plugins to customize horizontal
overflow in gr-formatted-text .markdown-html via CSS custom
property (--gr-formatted-text-markdown-html-overflow-x).

Release-Notes: skip
Change-Id: Id3c2ed62fffd0b766a781662eef31680c7c02648

polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts

diff --git a/polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts b/polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts
index 95b6851..917ed1d 100644
--- a/polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts
+++ b/polygerrit-ui/app/elements/shared/gr-formatted-text/gr-formatted-text.ts
@@ -119,7 +119,7 @@
           white-space: normal;
           /* prose will automatically wrap but inline <code> blocks won't and we
            should overflow in that case rather than wrapping or leaking out */
-          overflow-x: auto;
+          overflow-x: var(--gr-formatted-text-markdown-html-overflow-x, auto);
           overflow-wrap: break-word;
         }
       `,