Have RegexInterpreter work over ReadOnlySpan<char> instead of strings.

[joperezr]

Contributes to #59629

After chatting with @stephentoub offline, the approach we want to take to add support of spans on Regex will start by having the current engines move to working over Spans instead of strings. Once we have moved all engines do this, then we can add public surface area that takes in user spans, as opposed to strings.

This first PR is making it so the RegexInterpreter engine no longer uses the runtext string, other than in FindFirstChar and Go methods for getting the actual Span out, which is used in the rest of the implementation.

cc: @stephentoub@danmoseley

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Issue Details

---

Contributes to #59629

After chatting with @stephentoub offline, the approach we want to take to add support of spans on Regex will start by having the current engines move to working over Spans instead of strings. Once we have moved all engines do this, then we can add public surface area that takes in user spans, as opposed to strings.

This first PR is making it so the RegexInterpreter engine no longer uses the runtext string, other than in FindFirstChar and Go methods for getting the actual Span out, which is used in the rest of the implementation.

cc: @stephentoub@danmoseley

Author:	joperezr
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	-

[stephentoub]

And we're off :)