The docs didn't state the `v128.extract_lane`'s `idx` arg needs to compile time constant

### Bug description

as the name suggested, the docs entry for `v128.extract_lane` didn't state that the `idx` argument need to be compile time constant

### Steps to reproduce

1. try to compile this
```
let a = 0
v128.extract_lane<u8>(v128.splat<u8>(0), a)
```
2. compile error thrown

### AssemblyScript version

not sure if relevant but 0.27.30