Remove the goroutines from the shortcode lexer

Remove the goroutines from the shortcode lexer

It was clever, but storing the items in a slice is faster -- and it gives room to more goroutines in other places.

```bash
benchmark                     old ns/op     new ns/op     delta
BenchmarkShortcodeLexer-4     180173        79614         -55.81%

benchmark                     old allocs     new allocs     delta
BenchmarkShortcodeLexer-4     309            328            +6.15%

benchmark                     old bytes     new bytes     delta
BenchmarkShortcodeLexer-4     35456         47008         +32.58%
```