feat(dom-spectator): add elem query with root opt for DOMSelector (#273)

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• The commit message follows our guidelines: https:/ngneat/spectator/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

You can use DOMSpectator methods query(), queryAll() and queryLast() with param root: true to get HTML element finding it in document.
But it works ONLY for string selectors (like <title> or .some-class etc.).
I've got cases and there is would be great to have the possibility to pass selector which are instances of DOMSelector class (like byText, byAltText etc., custom DOMSelector selectors).

Issue Number: #273

What is the new behavior?

Works as previously but also allow to use DOMSelector instances as selector querying with root: true param.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information