Workflow for Windows nmake builds

[apnadkarni]

Adds a workflow for Windows using the nmake based build system including build, install and test steps.

[apnadkarni]

I think I'm done with this. Both Tcl and curl are cached now so subsequent runs are much faster. Please review and merge.

[bovine]

Great work. What would you think about renaming CURLDEPSDIR to ZLIBDIR, since it is currently only used to find zlib?

[apnadkarni]

The original reason for CURLDEPSDIR was that there are multiple additional dependencies depending on what options curl is built with that require additional libraries. The curl (as opposed to the vcpkg version of it) visual c++ build expects these to be in a single directory "deps" (see winbuild/readme.md in the curl sources). CURLDEPSDIR reflects that so at some point building tclcurl with the additional options (and subsequent additional dependencies) will not require multiple "ZLIBDIR" like options to be set.

Now vcpkg unfortunately uses a different directory layout which will complicate matters if we ever get to that point of additional options but that's a bridge to cross at some later point.

Hope that is clearer than mud :-)