Auto merge of #18243 - krobelus:clamp-position-character, r=Veykril

Clamp Position::character to line length

LSP says about Position::character

> If the character value is greater than the line length it defaults back to the line length.

but from_proto::offset() doesn't implement this.

A client might for example request code actions for a whole line by sending
Position::character=99999.  I don't think there is ever a reason (besides laziness) why the
client can't specify the line length instead but I guess we should not crash but follow protocol.

Not sure how to update Cargo.lock (lib/README.md doesn't say how).

Fixes #18240

Author: bors