Add initial locator foundation

[kblok]

Summary

• add the initial ILocator contract and concrete Locator wrapper
• expose Locator(string selector) on IPage and IFrame, plus Page and Frame implementations
• add minimal locator coverage for page entry points, frame entry points, and chaining/nth selection

Notes

• this is an initial locator foundation, not full parity for locator semantics yet
• strict mode and richer locator composition remain follow-up work

Verification

• dotnet build ./src/PlaywrightSharp/PlaywrightSharp.csproj --no-restore
• dotnet build ./src/PlaywrightSharp.Tests/PlaywrightSharp.Tests.csproj --no-restore was started in the sandbox but did not complete; the only emitted output before timeout was an existing NU1902 warning for SixLabors.ImageSharp 3.1.7