perf: use `final` strings rather than getters

[adil192]

This PR swaps out getters for final strings in the generated strings.g.dart file, e.g.

-   String get pages => 'Pages';
+   late final String pages = 'Pages';

Reasoning:

• Might unlock some performance optimizations in the dart compiler (just speculating here)
• Semantically, final fields represent that these strings don't change (in a given locale) more clearly than getters
• Note that fields are marked as late final not just final to avoid an error about implicit this references:

  lib/i18n/strings.g.dart:700:34: Error: Can't access 'this' in a field initializer to read '_root'.
  final String a = 'Tap on the "${_root.profile.quickLinks.deleteAccount}" button above';
                                  ^^^^^

I'm interested to see what you think about this (whether it's necessary or will make any difference) since this is just speculation

[Tienisto]

Hello, slang actually had final earlier. The reason why getters are used is because hot-reloading didn't work. You can try to update the i18n file, then generate, and then the strings should be updated.