Allow setting user (postfix)-snippet definitions

[TheSirC]

As described in the rust-analyzer codebase here, as far as I could find, there is no way for a rustic user to set rust-analyzer.completion.snippets.custom to allow for some custom snippet that would hook into rust-analyzer's parsing.

[psibi]

That should be part of the language server's client implementations eg: https://emacs-lsp.github.io/lsp-mode/page/lsp-rust-analyzer/#rust