[LSL] Add support for includes being relative to workspace root.

It would be useful to be able to include files from workspace root by formating the include with a leading `/`

## Example

Given the directory structure

```dir
workspace
├─ libs/
│  └─ lib.lsl
└─ src/
   └─ main.lsl
```

If `main.lsl` can use the following include
```lsl
#include "/libs/lib.lsl"
```
To reference `lib.lsl`