[clang-tidy] add option to allow functional casting to `modernize-avoid-c-style-cast`

```cpp
static char s_c = char(1);
```

```
:1:19: warning: C-style casts are discouraged; use static_cast [modernize-avoid-c-style-cast]
1 | static char s_c = char(1);
| ^~~~
| static_cast
```

https://godbolt.org/z/G47TxjWfq

The message is also misleading as this is clearly not a "C-style cast".

Start by reproducing the warning from the issue's functional-cast example, using the linked Compiler Explorer case if useful. Then locate the clang-tidy implementation and tests for modernize-avoid-c-style-cast; done means an option permits functional casts and the diagnostic no longer misleadingly calls them C-style casts.

Written by the indexing model from the issue text.