Implement WGSL template list discovery

[jimblandy]

Or at least, not without parentheses. The following is valid WGSL, but Naga rejects it, even on the const-expressions-2 branch:

$ cat expr-array-length.wgsl 
var<workgroup> buffer: array<u32, 1024 * 1024>;
$ naga expr-array-length.wgsl 
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `/home/jimb/rust/naga/target/debug/naga expr-array-length.wgsl`
error: expected '>', found '*'
  ┌─ expr-array-length.wgsl:1:40
  │
1 │ var<workgroup> buffer: array<u32, 1024 * 1024>;
  │                                        ^ expected '>'

Could not parse WGSL
$ 

If we change it as follows, Naga accepts it:

var<workgroup> buffer: array<u32, (1024 * 1024)>;

The problem here is that Naga doesn't yet implement template list discovery, so it can't tell that the > in array<u32, 1024 * 1024> is the closing bracket of a template list, rather than a > operator in an expression.