Support :i(ignorecase) / :m(ignoremark) with Str.substr-eq
Support :i(ignorecase) / :m(ignoremark) with Str.substr-eq

- on MoarVM, uses nqp::indexic / indexim / indexicim ops
- other backends use foldcase logic for :i(ignorecase)
- dies on other backends trying to use :m(ignoremark)

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