Assert that the vim.lsp.config function exists before calling it

[antoineco]

The plugin/blink-cmp.lua script added in af1febb currently doesn't check whether vim.lsp.config exists, which causes an error on startup if the Neovim nightly doesn't have this feature (ref. #897 (comment)). There is a broad range of development versions of Neovim 0.11 used in the wild, so solely checking for the nvim-0.11 feature isn't enough.

I considered two alternative implementations for this:

1. Skip the vim.fn.has('nvim-0.11') check entirely since it is not technically required.
   -> I left it on purpose because it has the merit of adding explicitness to the script.
2. Use pcall().
   -> Works equally well, but checking for the existence of the function is enough here.

[Saghen]

Thanks!