Make _ThreadLocal.value public under OpenSwiftUI SPI

Expose the value property of _ThreadLocal as public API under the OpenSwiftUI SPI to allow
access from the OpenSwiftUI framework while keeping it hidden from general public API.

Author: Kyle-Ye

Sources/OpenObservation/ThreadLocal.swift

@@ -19,7 +19,8 @@ func _tlsGet() -> UnsafeMutableRawPointer?
 func _tlsSet(_ value: UnsafeMutableRawPointer?)
 
 struct _ThreadLocal {
-  static var value: UnsafeMutableRawPointer? {
+  @_spi(OpenSwiftUI)
+  public static var value: UnsafeMutableRawPointer? {
     get {
       return _tlsGet()
     }
@@ -47,7 +48,8 @@ import Bionic
 #endif
 
 struct _ThreadLocal {
-  static var value: UnsafeMutableRawPointer? {
+  @_spi(OpenSwiftUI)
+  public static var value: UnsafeMutableRawPointer? {
     get {
       #if canImport(Darwin)
       return pthread_getspecific(key.key)