Add Most Wanted Features entries for <dialog>

Ports 0b21f2ecb1 to v3

Refs https://bugzil.la/840640
Refs https://webkit.org/b/84635

[skip sauce]

docs/_data/browser-features.yml

@@ -28,6 +28,16 @@
   origin: >
     Bootstrap#20143
 
+-
+  browser: >
+    Firefox
+  summary: >
+    Implement the HTML5 [`<dialog>` element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dialog)
+  upstream_bug: >
+    Mozilla#840640
+  origin: >
+    Bootstrap#20175
+
 -
   browser: >
     Chrome
@@ -57,3 +67,13 @@
     WebKit#64861
   origin: >
     Bootstrap#19984
+
+-
+  browser: >
+    Safari
+  summary: >
+    Implement the HTML5 [`<dialog>` element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dialog)
+  upstream_bug: >
+    WebKit#84635
+  origin: >
+    Bootstrap#20175