Explicit variant patterns for match expressions

The match expression in Clarity would benefit from supporting explicit variants for the match patterns, instead of just symbols to bind for each branch. This will also make the syntax more flexible, ready to adapt to advances like the native enum types proposed in stacks-network/reference#9.

Here is an example of explicit variant patterns to match on response type, with branches for the ok and err variants:

(match res
   (ok value) (+ value 1)
   (err code) code)

This replaces the current syntax:

(match res
   ok-value (+ ok-value 1)
   err-code err-code)

Note that in the current syntax, developers typically will use naming to signal the variant for each of the branches, but it can be obfuscated as it is the order of the branches not the name that matters. In contrast, with explicit variant patterns, the intent for each branch is apparent by default.

See also the proposed if-let construct:
https://github.com/blockstack/stacks-blockchain/issues/1403

Read the linked Clarity match documentation and compare the current and proposed pattern examples in this issue. Determine the language and parser entry points that implement match expressions, then define completion as supporting explicit ok and err variant patterns while preserving branch behavior and compatibility with the existing syntax.