CMakePM: Add support for "$ENV{" code completion and help

CMake has a few environment variables that are documented and this
commit adds support for them.

Change-Id: Iaa31bdc97b343581fcf519d19e66bc6ce8ace150
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/cmakeprojectmanager/cmaketool.cpp

@@ -288,6 +288,8 @@ CMakeKeywords CMakeTool::keywords()
             {"Help/variable", m_introspection->m_keywords.variables},
             // Policies
             {"Help/policy", m_introspection->m_keywords.policies},
+            // Environment Variables
+            {"Help/envvar", m_introspection->m_keywords.environmentVariables},
         };
         for (auto &i : introspections) {
             const FilePaths files = cmakeRoot.pathAppended(i.helpPath)