fix(playwright): allow include/exclude chaining when given a string by the user

[Zidious]

allow chaining of include/exclude when passed a string by the user

example:

 const results = await new AxeBuilder({ page })
        .exclude('.exclude')
        .exclude('.exclude2')
        .analyze();

closes issue: #389

[stephenmathieson]

By the title of this PR, I expected the change to be return this, not to allow multiple exclude/include selectors to be added 🤷

[stephenmathieson]

LGTM.

May want @michael-siek to OK this before it lands though. I'll leave the security review to him.