8124: Add basic lifetime completion r=Veykril a=Veykril
This adds basic lifetime completion, basic in the sense that the completions for lifetimes are only shown when the user enters `'` followed by a char. Showing them when nothing is entered is kind of a pain, as we would want them to only show up where they are useful which in turn requires a lot of tree traversal and cursor position checking to verify whether the position is valid for a lifetime. This in itself doesn't seem too bad as usually when you know you want to write a lifetime putting `'` to ask for lifetime completions seems fine.
~~I'll take a look at whether its possible to lift the restriction of having to put a char after `'`.~~ This actually already works so I guess this is the clients responsibility, in which case VSCode doesn't like it.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
