[FEA] Avoid regexp cost in string_split for escaped characters

[andygrove]

Is your feature request related to a problem? Please describe.
In GpuStringSplit we detect whether the provided pattern is a simple string or a regular expression so that we can call the appropriate cuDF method. There is an optimization that we could implement to avoid the cost of regexp in some cases. For example, given the pattern \. we could transpile to . and use the non-regex cuDF method.

Describe the solution you'd like
As described above.

Describe alternatives you've considered
None

Additional context
N/A