[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/BUILD.bazel

@@ -1,8 +1,21 @@
 load("@build_bazel_rules_swift//swift:swift.bzl", "swift_library")
 
+swift_library(
+    name = "FlexBuffers",
+    srcs = glob([
+        "Sources/FlexBuffers/**/*.swift",
+        "Sources/Common/*.swift",
+    ]),
+    module_name = "FlexBuffers",
+    visibility = ["//visibility:public"],
+)
+
 swift_library(
     name = "swift",
-    srcs = glob(["Sources/FlatBuffers/*.swift"]),
+    srcs = glob([
+        "Sources/FlatBuffers/*.swift",
+        "Sources/Common/*.swift",
+    ]),
     module_name = "FlatBuffers",
     visibility = ["//visibility:public"],
 )