Bug report: Utils.parseEscapedChars backslash matching

[pro-wh]

Describe the bug
Currently Utils.parseEscapedChars can match up to three backslashes in a replacement, which I think is unintended. As an example problem case, \\\\\\ unescapes to \\\\ instead of \\\. The reproduction link below does a round trip with escape, then unescape.

Following #582, I think we don't need the leading (\\)? and if (a === "\\") anymore. If there's an escaped backslash, it'll match with b equal to \, and whatever comes after will not be affected by the replace operation.

To Reproduce

• Version: 8.34.2
• Compile time: 28/06/2019 16:17:13 UTC
• User-Agent:
  Mozilla/5.0 (X11; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0
• Link to reproduce

Expected behavior
The output should be \\\.