feat: Error on missing `generics` parameter in `entry_points` macro

[jawoznia]

Additionally there is some refactor done. Some code could be reused. Also, MsgVariants generated some code it should not.

[jawoznia]

Kudos to @abefernan for pointing to the missing error ;)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 85 lines in your changes missing coverage. Please review.

Project coverage is 85.33%. Comparing base (d4c94ca) to head (8f6d1f6).

Files	Patch %	Lines
sylvia-derive/src/message.rs	0.00%	68 Missing :warning:
sylvia-derive/src/parser/entry_point.rs	0.00%	10 Missing :warning:
sylvia/tests/ui/macros/entry_points.rs	0.00%	6 Missing :warning:
sylvia-derive/src/lib.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #405 +/- ## ========================================== + Coverage 85.15% 85.33% +0.17% ========================================== Files 41 42 +1 Lines 2911 2905 -6 ========================================== Hits 2479 2479 + Misses 432 426 -6 ```

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[jawoznia]

397