Add license and copyright information for external dependencies

This is being added straight to 4.2, prior to the `make license` command
which will use this to generate a more complete license file.

Licenses information and ambiguities worked with Dalai Felinto.

Part of !129018.

extern/fast_float/README.blender

@@ -1,7 +1,8 @@
 Project: fast_float
 URL: https://github.com/fastfloat/fast_float
-License: MIT
+License: SPDX:MIT
 Upstream version: 5.0.0 (f5a3e77, 2023 May 25)
+Copyright: "Copyright 2023 The Wuffs Authors; Copyright (c) 2021 The fast_float authors"
 Local modifications:
 
 - Took only the fast_float.h header and the license/readme files