eslint warn on non null assertions

Summary:
Let's ban the non null assertion syntax `maybeNull!` as it is unsafe and can cause unexpected behavior. Ideally this should be an error or require a comment explaining why the value cannot be nullish. Though, there are 468 occurrences around the codebase and it is not worth it to manually go around fixing it now. Thus I used a warn to still warn developers to avoid the use where possible.

Last week I spend over an hour on debugging something where the code was null-asserted as the developer thought the value could never be null while in reallity it could. Adding a runtime check in this case sounds reasonable.

rule docs
https://typescript-eslint.io/rules/no-non-null-assertion/

Reviewed By: nikoant

Differential Revision: D45773327

fbshipit-source-id: 9e8a40af353ce979f469ffaedd8e777d72500dab

desktop/.eslintrc.js

@@ -220,6 +220,7 @@ module.exports = {
             leadingUnderscore: 'allow',
           },
         ],
+        '@typescript-eslint/no-non-null-assertion': 'warn',
       },
     },
     {