Update third-party/readme and license after adding support for mimalloc (#27088)

Updates the third-party/readme and license after adding support for
mimalloc

[Reviewed by @jhh67]

Author: jabraham17

LICENSE

@@ -49,6 +49,7 @@ The Chapel implementation is composed of two categories of code:
      libfabric        portable networking library                  BSD
      libunwind        used for runtime stack tracing               MIT
      llvm             CLANG C parsing/optional back-end compiler   Apache 2.0
+     mimalloc         alternative memory allocator                 MIT
      qthread          alternative lightweight tasking option       new BSD
      re2              optional regular expression parsing library  new BSD
      utf8-decoder     used for runtime UTF-8 string decoding       MIT

third-party/README

@@ -109,6 +109,18 @@ llvm/
   See also: llvm/llvm-src/README.txt and llvm/llvm-src/tools/clang/README.txt
 
 
+mimalloc/
+  Summary: mimalloc is an alternative allocator to with an emphasis on
+           multithreaded performance and reducing fragmentation
+
+  License: mimalloc is released under the MIT License (see
+           mimalloc/mimalloc-src/LICENSE)
+
+  Website: https://microsoft.github.io/mimalloc/
+
+  See also: mimalloc/README and mimalloc/mimalloc-src/README
+
+
 qthread/ 
   Summary: the Qthreads user-level tasking package developed by Sandia
            National Laboratory.