Spurious warnings about attributes are given by GCC < 9.1. See bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89325

Raise minimum compiler versions to those that support c++17.

The `TARGET` argument is an `INTERFACE` library which does not end up in
the build graph as something that can be requested of `make` or `ninja`
or works as a dependency of a custom target. Instead, make a custom
target that can be requested at build time by the user and depended upon
by other custom targets in the build.

- Adds `VTK_WEBASSEMBLY_EXCEPTIONS` cmake setting to
    enable exceptions at compile+link time with `-fexceptions`.
- This setting will allow the C++ unit tests to print additional stack traces
    when an uncaught exception is thrown, at the expense of greater binary size and slower execution.
- The `ENABLE_EXCEPTION_CATCHING` option requires a list of functions from which exceptions may be thrown.
    This commit allows all functions to throw exception catching with `DISABLE_EXCEPTION_CATCHING=0`
    as we cannot know all possible functions that could throw exceptions.

- the link options are now centralized in the `VTK::vtkWebAssemblyTestLinkOptions` target with `INTERFACE` visibility.
- this leverages the fact that all unit tests link with the `VTK::TestingCore` library.
- On emscripten platform, the asyncify link options are separately appended on the vtkRenderingWebGPUCxxTests target.

This file was first needed when each Python module was split into
an interface module (with the init function) and an implementation
library containing the Python.cxx files.  For the modern wrappers,
The InitImpl.cxx file can be folded into the Init.cxx file.