Allow `readCSV` to read a folder of CSVs

[Jolanrensen]

This is a common practice in Python, Excel and other data wrangling environments.

Currently, DataFrame.readCSV("path/to/directory") results in a strange DataFrame with a single column containing all the filenames in the directory (where the first file is the column name...): #508

What should happen is something like:

Path("path/to/dir").listDirectoryEntries("*.csv").map {
    DataFrame.readCSV(it.toFile())
}.concat()

This should work in the gradle/ksp/compiler plugin too

[Jolanrensen]

I must have misremembered, it also requires multiple steps to read multiple CSVs in, say, pandas: https://saturncloud.io/blog/how-to-import-multiple-csv-files-into-pandas-and-concatenate-into-one-dataframe/