Added new namingscheme option

Traditionally Meson has named output targets so that they don't clash with each other. This has meant, among other things, that on Windows Meson uses a nonstandard .a suffix for static libraries because both static libraries and import libraries have the suffix .lib.

There is now an option namingscheme that can be set to platform. This new platform native naming scheme that replicates what Rust does. That is, shared libraries on Windows get a suffix .dll, static libraries get .lib and import libraries have the name .dll.lib.

We expect to change the default value of this option to platform in a future major version. Until that happens we reserve the right to alter how platform actually names its output files.