[9.x] Added betweenFirst to Stringable

[yoeriboven]

I was getting unexpected results with the between method.

Str::between('[1] bla bla [2]', '[', ']');

// Expected: '1'
// Actual: '1] bla bla [2'

Looking at the tests this is actually the expected result and changing it would result in a breaking change.

I added the betweenFirst method to get the result I expected.

Str::betweenFirst('[1] bla bla [2]', '[', ']');

// '1'

between PR