Slicing a string literal

go 1.21, expr 1.15.8

I noticed slicing a string literal is giving an error:
```
"hello"[1:3]
```
> unexpected token Bracket("[") (3:8)

But slicing a string variable works:
```
let s = "hello"; s[1:3]
```
> "el"

I was hoping they would give the same output.