Implement transpilation of List.length into Spark code

[jonathanmaw]

i.e. like #794 but for List.length instead of List.maximum.

This involves:

• [ ] Write a simple as possible example using List.length
• [ ] Identifying the expression in Spark which does the same thing as a call to List.length
• [ ] Write transpilation rules for this in the Spark backend.
• [ ] Write tests to verify this functionality works as expected.
• [ ] Update documentation to reflect that List.length is supported.