feat: Allow negative start/end on std.slice

[He-Pin]

Motivation:
Fix: refs: #362

Modification:

1. Port he std.slice method in google/jsonnet to scala
2. Update the jsonnet test files
3. Extract the Util.slice method to support both std.slice and the usage in Evaluator.

[He-Pin]

Seems like the google/jsonnet is using the std.slice to handle both the evaluation of AST.Slice and std.slice function.