add firefox extension template

externs/webextension_polyfill/cookies/SameSiteStatus.hx

@@ -0,0 +1,8 @@
+package webextension_polyfill.cookies;
+
+/**
+	A cookie's 'SameSite' state (https://tools.ietf.org/html/draft-west-first-party-cookies).
+	'no_restriction' corresponds to a cookie set without a 'SameSite' attribute, 'lax' to 'SameSite=Lax',
+	and 'strict' to 'SameSite=Strict'.
+**/
+typedef SameSiteStatus = String;