Support custom user snippets during completion

[ericdallo]

As discussed here we could add a new feature on clojure-lsp:

Support for snippets and custom user snippets used during completion.

1. User could register via initializationOptions OR .lsp/config.edn a additional-snippets key/value:

{:additional-snippets [{:name "Wrap in let sexpr"
                        :snippet "(let [$1] $0$current-form)"}
                       {:name "Create private function"
                        :snippet "(defn ${1|^:private,-|} ${2:foo} [$3]\n  $0)"}]}

2. clojure-lsp should parse the know variables like $current-form and replace by the current selected form finding it via row and column provided on completionContext when completing.