fix: wrap templateFilter function

[danielbayley]

This wraps the given templateFilter function, simplifying things so that the first source path iteration is not the directory itself. Otherwise, the package consumer is forced to always write extra logic, just to account for this… For example—something like:

const source = import.meta.dirname
const templateFilter = path => path !== source && path.endsWith(".ext")
const fixture = await createFixture(source, { templateFilter })

vs

const templateFilter = path => path.endsWith(".ext")

[privatenumber]

Looks good but seems like a breaking change. Will hold for a bit.