Export getTextContent helper

[jvliwanag]

Exporting getTextContent is particularly useful for this kind of assertion:

expect(findManyRows().map((r) => within(r).findManyCells().map((c) => getTextContent(c))).toStrictEqual([
  ["foo", "FOO"],
  ["bar", "BAR"],
]);

Workaround is to loop through everything running expect on every ReactTestInstance encountered, but test errors are harder to decipher.

The dom version of the testing-library already has a precedent here.

[mdjastrzebski]

@jvliwanag We certainly could export this function. I'll try to squize PR for it in the next couple of days.