Feature request: lower/upper case assertions

Description

Please introduce lowercase and uppercase assertions for strings or chars, for example:

// Arrange
var input = "A";

// Act
var actual = input.ToUpper();

// Assert
actual.Should().BeUpper();
actual.Should().NotBeLower();

[jnyrup]

Culture invariance is still relevant to behave identically on a Turkey Test no matter the current culture on the testing machine.

[weitzhandler]

Or having an overload that supports BeUpper(CultureInfo), which can then be used with CultureInfo.InvariantCulture.