projects: fix Windows project 'clean' function

- Fix generate.bat "-clean" option.

- Change version template substitutes to match old files, eg go back to
  using format version "11.00" instead of "11.0".

- Limit the vcxproj filters file types that are filtered to c, h, rc.

- Get rid of the tmpl extension from template files and add a README
  to the tmpl directory explaining the purpose of the files.

- gitignore VCxx directories entirely rather than individual file types.

- Do not remove the VC directories during clean, instead remove just the
  generated project files.

Removing the VC directories has the unwanted behavior of removing files
other than those generated. Visual Studio will generate its own
preference files (like if you have some debug arguments in your .suo)
and those files sit in the VC directories. We ignore those files since
they are the user's files and should not be deleted. Also the user may
have their own untracked files that we shouldn't be deleting.

Follow-up to 57d349fe which consolidated the project templates.

Assisted-by: Viktor Szakats

Closes https://github.com/curl/curl/pull/18412

projects/Windows/.gitignore

@@ -2,9 +2,6 @@
 #
 # SPDX-License-Identifier: curl
 
-*.ncb
-*.opensdf
-*.sdf
-*.suo
-*.user
-*.vcxproj
+VC10/
+VC11/
+VC12/