Provide a warning when using a default (recursive workspace includes) configuration with a large project #12817

If a custom configuration provider (i.e. CMake Tools) does not register (or for files not associated with a CMakeLists.txt), the default configuration used by the C/C++ Extension is to recursively consider every subdirectory under the workspace folder as an include path. This is equivalent to having the following in the configuration:

            "includePath": [
                "${workspaceFolder}/**"
            ],

For large, complex project workspaces, particularly those containing various subprojects (potentially with some of the same filenames as other projects) this may or may not result in a reasonable configuration.

It may help to provide a warning when using a default (recursive workspace includes) configuration and an excessive number of files or include directories are found, or files or directories with duplicate names are found. A warning could refer users to guidance on how to configure the extension, perhaps pointing them directly to documentation specific to likely scenarios for existing projects (which likely have a build system, or otherwise provide information on how to properly configure a build). Without this, users may assume that the extension is simply failing to work properly and not discover that some configuration may be necessary.