[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"]
     }],

gulpfile.js

@@ -1288,6 +1288,10 @@ function preprocessDefaultPreferences(content) {
   return licenseHeader + "\n" + MODIFICATION_WARNING + "\n" + content + "\n";
 }
 
+function replaceMozcentralCSS() {
+  return replace(/var\(--(inline-(?:start|end))\)/g, "$1");
+}
+
 gulp.task(
   "mozcentral",
   gulp.series(
@@ -1357,10 +1361,12 @@ gulp.task(
 
         preprocessCSS("web/viewer.css", defines)
           .pipe(postcss([autoprefixer(MOZCENTRAL_AUTOPREFIXER_CONFIG)]))
+          .pipe(replaceMozcentralCSS())
           .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
 
         preprocessCSS("web/viewer-geckoview.css", defines)
           .pipe(postcss([autoprefixer(MOZCENTRAL_AUTOPREFIXER_CONFIG)]))
+          .pipe(replaceMozcentralCSS())
           .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
 
         gulp

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*/