Develop expression language for executing rules

[ubdbra001]

bidsschematools (BST) has tooling for parsing the Javascript-ish rules in the schema, but the validator currently doesn't parse rules and check these parsed rules.

This issue should address the following:

• Parse the rules (using or possibly reimplementing the BST tooling)
• Execute the selector rules on specific file contexts to identify which checks should be run for a specific file
• Execute the checks against that file (and report errors)?

Suggestion was to start by looking at the pyparsing package for examples.

Is this roughly correct @effigies? What did I miss?

[effigies]

Looks right

[ubdbra001]

Use schema.meta.expressiontests as starting point for parsing and executing, they don't need an evaluation context.