adds changes for regex constraint implementation

[desaikd]

Issue #9 #10

Description of changes:
This PR works on adding implementation of regex constraint.

Grammar:

<REGEX> ::= regex: <STRING>
          | regex: i::<STRING>
          | regex: m::<STRING>
          | regex: i::m::<STRING>

Ion Schema specification:
https://amzn.github.io/ion-schema/docs/spec.html#regex

List of changes:

• adds regex crate dependency
• adds Regex enum variants for IslConstraint and Constraint
• adds RegexConstraint implementations
  (Note: Not all features of regex are supported as per Ion schema specification hence there is a preprocessing done on the regex to allow only the supported features)
• reference for preprocessing regex is as per ion-schema-kotlin implementation.
• adds validation logic for regex
• adds ViolationCode for RegexMismatched

Tests:
added unit tests for regex implementation.

• adds tests for programmatically creating regex constraint
• adds tests for loading a schema using regex constraint
• adds validation tests for regex constraint

[popematt]

This isn't a reflection on your code at all, but it's disappointing that we need handwritten code for validating the regex pattern. Maybe there's a crate that we could use to generate a parser from a grammar that can validate the regex pattern (e.g. Pest). 🤷‍♂️

That being said, I recommend you follow Clippy's advice about the loop. Otherwise it looks good to me.

[desaikd]

I will open up an issue to investigate on whether something like Pest could be used here.
Thanks for the suggestion!

This isn't a reflection on your code at all, but it's disappointing that we need handwritten code for validating the regex pattern. Maybe there's a crate that we could use to generate a parser from a grammar that can validate the regex pattern (e.g. Pest). 🤷‍♂️

Issue reference: #97