Implement LinkML structured_pattern

[ddooley]

See https://github.com/linkml/linkml-runtime/blob/a49927d45eb3584f85921fdba6d5c1cd75f54585/tests/test_utils/input/pattern-example.yaml and https://github.com/linkml/linkml/issues/176 for examples of regular expressions and string substitition from structured_pattern converted into compiled regular expression in pattern field. Python version of algorithm for string substitution at bottom of https://github.com/linkml/linkml/issues/674