[Dexter] Add basic structured script parsing (#193710)
See PSA:
https://discourse.llvm.org/t/psa-planned-changes-to-dexter/90402
This patch begins adding support for "structured scripts" to Dexter,
starting with some of the core classes and the ability to parse script
files. This patch does not add the ability to actually run scripts, or
any of the underlying functionality required to do so.
NB: This patch adds a dependency on PyYAML, which is specified in a new
requirements.txt file.
