Add linting to frontend (#4341)

# Description of Changes
There's no current linter running over our TypeScript code, which means
we've got a bunch of dead code and other code smells around with nothing
notifying us. This PR adds ESLint with the typescript-eslint plugin and
enables the recommended settings as a starting point for us.

I've disabled all of the failing rules for the scope of this PR, just to
get linting running without causing a massive diff. I'll follow up with
future PRs that enable the failing rules one by one.

Also updates our version of TypeScript, which introduces a new type
error in the code (which I've had to fix)

.editorconfig

@@ -24,7 +24,7 @@ indent_size = 2
 insert_final_newline = false
 trim_trailing_whitespace = false
 
-[{*.js,*.jsx,*.ts,*.tsx}]
+[{*.js,*.jsx,*.mjs,*.ts,*.tsx}]
 indent_size = 2
 
 [*.css]