Add docs and tests for Field Arrays

[crutchcorn]

Right now, we do support Field Arrays, but their usage is not clear and I have low confidence as a maintainer that we won't break them in the future. Let's update docs and tests for field arrays so that we're able to ship this more confidently.

[Christian24]

I just tested this in my prototype. How is the API meant to work? I just iterated over the array and built the field name with the index aka

{ name: `employees[${index}].color` }

This seems to work fine, but DeepKeys does not support it. Is this how you envision the API for FieldArrays though, @crutchcorn?

Btw my updated prototype is here: https://github.com/Christian24/tanstack-form-lit-prototype/tree/arrays

[Christian24]

Upon further investigation it seems correct names look as follows:

{ name: `employees.${index}.color` }

This works and types also work for this.

[Christian24]

Interestingly, there seems to be some documentation on the website already: https://tanstack.com/form/latest/docs/guides/basic-concepts#array-fields

I did do a quick find usages on index though and Webstorm said it was used nowhere. So, I have no idea if these examples are correct.

[Christian24]

There are also already tests present, so maybe this can be closed @crutchcorn

[crutchcorn]

Yea this deffo can be closed, my bad