[stdlib] Fix implementation of Unicode text segmentation for word boundaries

Carefully overhaul our word breaking implementation to follow the recommendations of Unicode Annex #29. Start exposing the core primitives (as well as `String`-level interfaces), so that folks can prototype proper API for these concepts.

- Fix `_wordIndex(after:)` to always advance forward. It now requires its input index to be on a word boundary. Remove the `@_spi` attribute, exposing it as a (hidden, but) public entry point.
- The old SPIs `_wordIndex(before:)` and `_nearestWordIndex(atOrBelow:)` were irredemably broken; follow the Unicode recommendation for implementing random-access text segmentation and replace them both with a new public `_wordIndex(somewhereAtOrBefore:)` entry pont.
- Expose handcrafted low-level state machines for detecting word boundaries (_WordRecognizer`, `_RandomAccessWordRecognizer`), following the design of `_CharacterRecognizer`.
- Add tests to reliably validate that the two state machine flavors always produce consistent results.

rdar://155482680

stdlib/public/core/StringIndexValidation.swift

@@ -400,20 +400,3 @@ extension _StringGuts {
       scalarAlign(validateInclusiveSubscalarIndex_5_7(i)))
   }
 }
-
-// Word index validation (String)
-extension _StringGuts {
-  internal func validateWordIndex(
-    _ i: String.Index
-  ) -> String.Index {
-    return roundDownToNearestWord(scalarAlign(validateSubscalarIndex(i)))
-  }
-
-  internal func validateInclusiveWordIndex(
-    _ i: String.Index
-  ) -> String.Index {
-    return roundDownToNearestWord(
-      scalarAlign(validateInclusiveSubscalarIndex(i))
-    )
-  }
-}