rubico/x/startsWith

```coffeescript
startsWith(prefix string)(value string) -> boolean
```
Determine whether a string starts with a prefix.

```javascript
console.log(startsWith('h')('hey')) // true
```

Look for the x/ directory in the codebase, which likely contains utility functions. Check existing similar functions (like endsWith) to understand the pattern. Implement startsWith to take a prefix string and return a function that checks if a given string starts with that prefix. Write a test in the test suite to verify the behavior matches the example.

Written by the indexing model from the issue text.