fix: In generate_mut_trait_impl, don't add a tabstop if the client does not support snippets

[ChayimFriedman2]

Fixes #20320.

We should have better snippet handling. I started working on that but got distracted by other things.

[DropDemBits]

cc #17332 (generate_constant is now the only misbehaving assist).

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We should have better snippet handling.
@ChayimFriedman2 curious about your thoughts, since I was the one who did the initial push away from representing snippets in-text.

[ChayimFriedman2]

@ChayimFriedman2 curious about your thoughts, since I was the one who did the initial push away from representing snippets in-text.

Yes, but a lot of assists still do it by text, we should switch everything to not use that (even assists which do not use the node editor, which is currently impossible for them), then escape snippet bits in the generated text.