[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/UnicodeBreakProperty.swift

@@ -13,7 +13,7 @@
 import SwiftShims
 
 extension Unicode {
-  internal enum _GraphemeBreakProperty {
+  internal enum _GraphemeBreakProperty: Sendable {
     case any
     case control
     case extend
@@ -86,7 +86,7 @@ extension Unicode {
 }
 
 extension Unicode {
-  internal enum _WordBreakProperty {
+  internal enum _WordBreakProperty: UInt8, Sendable {
     case aLetter
     case any
     case doubleQuote
@@ -105,8 +105,8 @@ extension Unicode {
     case singleQuote
     case wSegSpace
     case zwj
-    
-    init(from scalar: Unicode.Scalar) {
+
+    internal init(from scalar: Unicode.Scalar) {
       switch scalar.value {
       case 0xA ... 0xD,
            0x85,
@@ -122,7 +122,7 @@ extension Unicode {
         self = .regionalIndicator
       default:
         let rawValue = _swift_stdlib_getWordBreakProperty(scalar.value)
-        
+
         switch rawValue {
         case 0:
           self = .extend