Windows: Port icu4c; define cxx std flags for MSVC (#45547)

* Adds an MSBuild system + Builder to the icu4c package
* Adds custom install method as MSBuild system does not vendor an
  install target
* The cxxstd variant is not supported on Windows (there are no config
  options you use to tell the build system what cxx standard to
  build against), so the variant definition was updated to occur
  everywhere except Windows

Also, this commit defines the c/cxx..._flag properties of the MSVC
compiler (although they are not used by `icu4c` and not strictly
necessary to bundle with this PR).

lib/spack/spack/compilers/msvc.py

@@ -223,6 +223,30 @@ def get_oneapi_root(pth: str):
             )
         self.msvc_compiler_environment = CmdCall(*env_cmds)
 
+    @property
+    def cxx11_flag(self):
+        return "/std:c++11"
+
+    @property
+    def cxx14_flag(self):
+        return "/std:c++14"
+
+    @property
+    def cxx17_flag(self):
+        return "/std:c++17"
+
+    @property
+    def cxx20_flag(self):
+        return "/std:c++20"
+
+    @property
+    def c11_flag(self):
+        return "/std:c11"
+
+    @property
+    def c17_flag(self):
+        return "/std:c17"
+
     @property
     def msvc_version(self):
         """This is the VCToolset version *NOT* the actual version of the cl compiler