Add some code snippets

[aabounegm]

Snippets make it easier for the users to enter repeating code patterns. We should contribute some snippets in the extension to speed up development with Rzk.
Note that while snippets are generally provided by the VS Code wrapper, the LSP server can also provide snippets in the form of completion items. These would be more relevant for snippets that depend on the current context.

Some snippets I could think of:

• rzk1 ➡️ #lang rzk-1
• def ➡️ #define ${1:name} : ${2:type} := $0

Perhaps there are patterns that get repeated often in proofs, but I haven't written enough proofs myself to identify them (hence the help wanted label)