Add T-SQL parser in Go with AST types

[kyleconroy]

• Create ast package with core AST type definitions (Script, Batch,
  Statement, SelectStatement, QuerySpecification, etc.)
• Create parser package with Parse function that takes ctx and io.Reader
  and returns ast.Script
• Add MarshalScript function to serialize AST to JSON format matching
  the expected testdata format
• Add test using ZeroLengthFile testdata to verify JSON output
• Remove sample.sql and ast.json from root (cleanup)