[Firefox] Use float: inline-start/inline-end directly in MOZCENTRAL builds (PR 15968 follow-up)

Currently `float: inline-start/inline-end` is only supported in Firefox, see https://developer.mozilla.org/en-US/docs/Web/CSS/float#browser_compatibility, and in order to support other browsers we're thus forced to jump through some hoops.
This leads to slightly less nice code in the *built-in* Firefox PDF Viewer, and this patch attempts to improve the current situation:
 - Use Stylelint to forbid direct use of `float: inline-start/inline-end` in the CSS files, to prevent future bugs in the general PDF.js viewer.
 - Do a build-time replacement, only in MOZCENTRAL builds, to replace the CSS-variables with raw `float: inline-start/inline-end` instances.

web/viewer.css

@@ -17,10 +17,7 @@
 
 :root {
   --dir-factor: 1;
-  /*#if MOZCENTRAL*/
-  --inline-start: inline-start;
-  --inline-end: inline-end;
-  /*#else*/
+  /*#if !MOZCENTRAL*/
   --inline-start: left;
   --inline-end: right;
   /*#endif*/