[WIP] Add support for verus_spec formatting

[jaybosamiya-ms]

Closes #121

TODO list:

• Get a significant example file
• Update minimal parsing to prevent rustfmt from mangling verus_spec
• Set up unstable CLI flag to gate verus_spec syntax
• Set up a new phase for formatting verus_spec after the rustfmt pass
• Add a parser for verus_spec syntax (TBD: how do we share with the main verus.pest if we write a new .pest? would it be safe to update verus.pest with verus_spec-specific things instead?)
• Write the to-doc functionality for the verus_spec variants
• Test for and report warnings if line-numbering might be confusing