[Swift] Flexbuffers native swift port (#8577)

* Offical Swift port for FlexBuffers

This is the offical port for FlexBuffers within
swift, and it introcudes a Common Module where code
is shared between flatbuffers and flexbuffers.

Writing most supported values like maps, vectors,
nil and scalars into a flexbuffer buffer. And includes
tests to verify that its similar to cpp

* Reading a flexbuffer

Implementing reading from a flexbuffer, enabling
most of the buffers features, like most types, maps, vectors,
typedvectors, and fixedtypedvectors.

Currently, if an offset/object cant be read we default to a swift
nil instead of the default flexbuffers 'null' with all values.

* Fixes bazel breaking due to new project structure

Address warnings within the library

* Adds comment on why we added the code & properly enforce the amout of bytes needed

swift/Sources/FlexBuffers/Utils/BitWidth.swift

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2024 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+
+@usableFromInline
+enum BitWidth: UInt64, CaseIterable {
+  case w8 = 0, w16 = 1, w32 = 2, w64 = 3
+}
+
+extension BitWidth: Comparable {
+  @usableFromInline
+  static func < (lhs: BitWidth, rhs: BitWidth) -> Bool {
+    lhs.rawValue < rhs.rawValue
+  }
+
+  @inline(__always)
+  static func widthB(_ v: Int) -> BitWidth {
+    switch v {
+    case 1: return .w8
+    case 2: return .w16
+    case 4: return .w32
+    case 8: return .w64
+    default:
+      assert(false, "We shouldn't reach here")
+      return .w64
+    }
+  }
+
+  @inline(__always)
+  static func max(_ lhs: BitWidth, rhs: BitWidth) -> BitWidth {
+    if lhs.rawValue > rhs.rawValue { return lhs }
+    return rhs
+  }
+}