fix(lsp): tune completion word extraction for decorated labels (#29331)

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

runtime/lua/vim/lsp/completion.lua

@@ -145,8 +145,8 @@ local function get_completion_word(item)
       --    label: insert
       --
       -- Typing `i` would remove the candidate because newText starts with `t`.
-      local text = item.insertText or item.textEdit.newText
-      return #text < #item.label and text or item.label
+      local text = parse_snippet(item.insertText or item.textEdit.newText)
+      return #text < #item.label and vim.fn.matchstr(text, '\\k*') or item.label
     elseif item.insertText and item.insertText ~= '' then
       return parse_snippet(item.insertText)
     else