feat: add support for string-based custom queries

[paullewis]

Relevant issues: #5747, #5405, #4171.

[mathiasbynens]

Looking great so far! Left some thoughts, PTAL.

[paullewis]

I think that's all the feedback addressed. PTAL.

(@mathiasbynens I will namespace this under experimental as discussed when everything else is okay)

[paullewis]

Okay, these are the call sites. I'll go through and confirm that each one lands in the correct place to have custom query support.

• page.$(selector)
• page.$$(selector)
• page.$$eval(selector, pageFunction[, ...args])
• page.$eval(selector, pageFunction[, ...args])
• page.click(selector[, options])
• page.focus(selector)
• page.hover(selector)
• page.select(selector, ...values)
• page.tap(selector)
• page.type(selector, text[, options])
• page.waitFor(selectorOrFunctionOrTimeout[, options[, ...args]])
• page.waitForSelector(selector[, options])
• frame.$(selector)
• frame.$$(selector)
• frame.$$eval(selector, pageFunction[, ...args])
• frame.$eval(selector, pageFunction[, ...args])
• frame.click(selector[, options])
• frame.focus(selector)
• frame.hover(selector)
• frame.select(selector, ...values)
• frame.tap(selector)
• frame.type(selector, text[, options])
• frame.waitFor(selectorOrFunctionOrTimeout[, options[, ...args]])
• frame.waitForSelector(selector[, options])
• elementHandle.$(selector)
• elementHandle.$$(selector)
• elementHandle.$$eval(selector, pageFunction[, ...args])
• elementHandle.$eval(selector, pageFunction[, ...args])

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[paullewis]

Changed CLA to yes because all commits were by people who have signed the CLA.

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[paullewis]

Changed CLA to yes (again) because all commits were by people who have signed the CLA.

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