flow: Enable Types-First mode!

From Flow's perspective, we're fine to delay types-first until we're
on v0.143, which we'd normally take along with the RN v0.65 upgrade.

But, from experimentation, React Native v0.64's internal code seems
to have been written with Types-First, such that a few Flow errors
would show up if we tried to use RN v0.64 without Types-First
enabled. So, enable it, now that we can, and now that the RN v0.64
upgrade is on the horizon.

Fixes: zulip#4907

.flowconfig

@@ -113,5 +113,7 @@ module.file_ext=.js
 module.file_ext=.json
 module.file_ext=.ios.js
 
+types_first=true
+
 [version]
 ^0.128.0