engine: `'\\'` turn in to `''` when converting from AST to Logical plan

[DanCodedThis]

• This was found as part of the regexp_* function family
• However, this affects all strings
• Parser is fine with '\\' but Logical plans seem to remove the \\ to turn it to an empty string
• This also happens like this \\b -> b, etc (there are many other similar cases)
• The cure seems to be (a hacky way solution) is to replace \\ with \\\\ then when we get to the execution from the logical plan, we actually get \\ which is what we want
• Perhaps there is a setting in DF ctx config?
• This can be fixed probably in the expr planner, etc