feat: Assist to replace generic with impl trait
This adds a new assist named "Replace named generic with impl". It is the inverse operation to the existing "Replace impl trait with generic" assist.
It allows to refactor the following statement:
```rust
// 👇 cursor
fn new<T$0: ToString>(input: T) -> Self {}
```
to be transformed into:
```rust
fn new(input: impl ToString) -> Self {}
```
* adds new helper function `impl_trait_type` to create AST node
* add method to remove an existing generic param type from param list
Closes#14626
e963846656