Add more completion items for `impl` to handle common scenarios

[Natural-selection1]

Currently, the completion for "impl" only provides the basic "impl" snippet. While this is universally applicable, I believe it overlooks adaptation for common usage scenarios - which is precisely where code completion should excel. Therefore, I propose adding more relevant completion items.

According to the Rust Reference, I suggest adding these two items would be appropriate:

add_keyword("impl for", "impl $1 for $2 {\n    $0\n}");
add_keyword("impl<> for <>", "impl<$4> $1 for $2<$3> {\n    $0\n}");

However, I would like to gather opinions regarding:

1. The naming conventions for these two items
2. The cursor navigation order in the second snippet (the current version reflects what I consider to be a reasonable sequence)

The current proposals represent my initial thoughts on optimal implementation.