Prepare for moving datafusion-comet-spark-expr to core DataFusion repo

[andygrove]

What is the problem the feature request solves?

We would like to move the spark-expr crate upstream into DataFusion.

Before we can do this, we need to:

• Add tests that can run without a dependency on Apache Spark.
• Improve the documentation to explain how these expressions differ from the DataFusion
• Add an example showing how to register these functions with a DataFusion context
• Remove any references to Comet
• Add a README listing the expressions that are supported

Describe the potential solution

No response

Additional context

No response