Features of a pattern matching syntax

> **Note** The latest version of the design based on these principles can be found [here](https://github.com/dotnet/vblang/issues/337#issuecomment-601614253).

**What features / attributes should a VB.NET pattern matching syntax have?**

I propose the following:

* **Allow the same syntax for patterns in a boolean-returning expression (introduced with a dedicated keyword), as for patterns in a Case clause.** This enables patterns to be used as the condition of `If ... Then`, `Do While ...` blocks and the like.
* **Allows the pattern to introduce variables into the child scope** -- this will be either the `Case` block or the block following the match expression
* **Pattern-neutral**, not oriented specifically to a type-checking pattern
* **Allow `When` clauses** for additional conditions
* **Allows for nested patterns** -- certain pattern types can contain other patterns

I've written some examples, and a proposed spec in line with these goals [here](https://github.com/dotnet/vblang/issues/124#issuecomment-416292181). But the purpose of this issue is to discuss whether these goals are valid and relevant.