feat: add toHaveCount and toBeEmpty assertions

[alexC2K]

Summary

Adds two new locator assertions to LocatorAssertions in expect.ts, matching the Playwright API surface:

• toHaveCount(n) - polls until the number of elements matched by a locator equals n.
• toBeEmpty() - polls until a form element's value is an empty string.

Both assertions support .not negation and an optional { timeout } override, consistent with all other assertions in the file.
Also added unit tests for these two assertions.

Test plan

• expect(locator).toHaveCount(3) passes when count matches
• expect(locator).toHaveCount(5) throws ExpectError on mismatch (with short timeout)
• expect(locator).not.toHaveCount(5) passes when count differs
• expect(locator).toHaveCount(0) passes when nothing is found
• expect(locator).toBeEmpty() passes when value is ""
• expect(locator).toBeEmpty() throws when value is non-empty
• expect(locator).not.toBeEmpty() passes when value is non-empty
• All unit tests pass

[gmegidish]

@alexC2K thank you for this great addition! merging it in!