fix: support self-contained executables

[mxschmitt]

Should not be any big logical changes, just safeguards the access to assembly.CodeBase and falls-back to AppContext.BaseDirectory otherwise which is important for the self-contained executables.

#2255

[nohwnd]

This looks like it should work, would be nice to have a test that proves it works. But if this is not a main scenario for you it seems okay to keep as is.

[mxschmitt]

Tested on macOS and Windows. One thing to note is that playwright.ps1 does not work, this is because its a single executable and the ps1 is looking for Microsoft.Playwright.dll.

Workaround: Microsoft.Playwright.Program.Main(["install"]);

Will keep the issue open for now, but this patch should help the users already a lot.