add tree_string

[MrPowers]

Don't merge this till it's working.

Method that I tried to copy from Spark source code: https://github.com/apache/spark/blob/edf4ac4b518d0d69f7012ff5c0f1428fe45412ba/sql/api/src/main/scala/org/apache/spark/sql/types/StructType.scala#L399

Purpose: give users a better experience for analyzing diffs between two schemas with nested fields that are hard to read in the terminal.

[MrPowers]

Related issue: https://github.com/MrPowers/chispa/issues/88