regex validator support for negative MatchString

[neatcoder]

Terraform CLI and Framework Versions

TF: 1.7.5
framework: 0.12.0

Use Cases or Problem Statement

Since golang doesn't support lookahead and lookbehind regular expressions, it would be easier to have a solution in stringvalidator package for negative regex match. For example:
Validation to match any string that does not start with GITHUB_. Usually it is done with negative lookahead but golang does not support it. TF Framework only supports "match" pattern but there is no function provided o work with elimination or strings that don't match a pattern.

Proposal

New RegexNotMacthes function could be defined in stringvalidator package so that it approves a string that does not match a given regex pattern.

Additional Information

No response

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