feat: expectation matchers

[Ehesp]

Tracking issue identify progress of the various expectation matchers.

• expect(value)
• .not
• .toBe(value)
• .toEqual(value)
• .isNull
• .isNotNull - Dart equivalent of expect.anything()
• .toBeFalsy
• .toBeTruthy
• .toThrow(Error?)

String

• expect.stringToMatch(Pattern)
• expect.not.stringToMatch(Pattern)

Number

• .toBeGreaterThan(num)
• .toBeGreaterThanOrEqual(num)
• .toBeLessThan(num)
• .toBeLessThanOrEqual(num)

List

• expect.listContaining(List)
• expect.not.listContaining(List)
• .toContain(item)
• .toContainEqual(item)

Map

• expect.mapContaining(Map)
• expect.not.mapContaining(Map)
• .toHaveProperty(path, value?)

Future

• .resolves
• .rejects

[bartekpacia]

Are you planning to add Flutter-only matchers? I'm thinking about something like expect(find.text('Hi').toFindOneWidget().