fix: Support uppercase letters in gist link (#2696)

shared/embeds/Gist.js

@@ -2,7 +2,7 @@
 import * as React from "react";
 
 const URL_REGEX = new RegExp(
-  "^https://gist.github.com/([a-z\\d](?:[a-z\\d]|-(?=[a-z\\d])){0,38})/(.*)$"
+  "^https://gist.github.com/([a-zA-Z\\d](?:[a-zA-Z\\d]|-(?=[a-zA-Z\\d])){0,38})/(.*)$"
 );
 
 type Props = {|

shared/embeds/Gist.test.js

@@ -15,6 +15,12 @@ describe("Gist", () => {
         match
       )
     ).toBeTruthy();
+
+    expect(
+      "https://gist.github.com/ShubhanjanMedhi-dev/900c9c14093611898a4a085938bb90d9".match(
+        match
+      )
+    ).toBeTruthy();
   });
 
   test("to not be enabled elsewhere", () => {