[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.

.stylelintrc

@@ -8,6 +8,9 @@
 
     "block-no-empty": true,
     "declaration-block-no-redundant-longhand-properties": true,
+    "declaration-property-value-disallowed-list": {
+      "float": ["inline-start", "inline-end"]
+    },
     "length-zero-no-unit": [true, {
       ignore: ["custom-properties"]
     }],