Adding a new Locator strategy

All,
 I've created a github issue (https://github.com/w3c/webdriver/issues/1580),
but wanted to make sure I'm making this visible to the correct parties
(hopefully I am).

Again, I'm offering to do development and writing if there's no objections.

Thanks,
Matt

-- Github issue copy and pasted below to avoid confusion

Add a Locator Strategy <https://w3c.github.io/webdriver/#locator-strategies>
for locating an input-type element (<select>, <input>, etc and potentially
even non-input labellable elements
<https://www.w3.org/TR/html52/sec-forms.html#labelable-element>) by the
text of the label for that element.

In addition to being an *incredibly common* use case, this will encourage
improving the accessibility (a11y) of the markup - forms having labels is
one of the first checks by Chrome DevTools' Lighthouse, Firefox's Developer
Tools (Accessibility tab) or other accessibility checkers. These are all
based off the WCAG guidelines
<https://www.w3.org/WAI/WCAG21/quickref/?showtechniques=332#labels-or-instructions>

I'll can write it up more formally with a pull request if it seems
worthwhile.

(Originally created in SeleniumHQ/selenium#9342
<https://github.com/SeleniumHQ/selenium/issues/9342>)
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