Add queryElement (singular) API in addition to queryElements

[zcorpan]

In #1343 we added queryElements as a wrapper for WebDriver findElements, which would fail the test if no element was found.

In #1017 we tried to adopt this in the test for that PR, but there were multiple cases where it used findElement to get the first element. Using queryElements makes the test slightly slower (in theory) and the code more convoluted since it needs to get the first element of the resulting array.

  await (await t.context.queryElements(t, ex.buttonSelector))[0].sendKeys(Key.ENTER);

vs (proposed)

  await (await t.context.queryElement(t, ex.buttonSelector)).sendKeys(Key.ENTER);

cc @smhigley @jongund @nschonni

[smhigley]

Is this primarily for author convenience, so both functions have the same naming convention? I think we could just alias queryElement to findElement.