Problem: CompletionItem in lsp spec mentioned the deprecated attribute
Solution: when item has deprecated attribute set hl_group to DiagnosticDeprecated
in complete function
Although the built-in pum completion mechanism will filter anyway on the
next input it is odd if the initial popup shows entries which don't
match the current prefix.
Using fuzzy match on the label/prefix is compatible with
`completeopt+=fuzzy` and also doesn't seem to break postfix snippet
cases
Closes https://github.com/neovim/neovim/issues/29287
Problem:
For snippets lsp.completion prefers the label if it is shorter than the
insertText or textEdit to support postfix completion cases but clangd
adds decoration characters to labels. E.g.: `•INT16_C(c)`
Solution:
Use parse_snippet on insertText/textEdit before checking if it is
shorter than the label.
Fixes https://github.com/neovim/neovim/issues/29301
Problem: Completion side effects not working randomly.
Solution: When creating the table of LSP responses, the table index
was used, but this is not the same as the actual client_id, so it was changed
to use the client_id directly.
The `complete()` mechanism matches completion candidates against
the typed text, so strict pre-filtering isn't necessary.
This is a first step towards supporting postfix snippets (like
`items@insert` in luals)
