bug: .dat savefig uses PNG backend

[krystophny]

Summary

Saving with a .dat extension does not switch to the ASCII backend. get_backend_from_filename only recognizes .png, .pdf, and .txt, so .dat falls back to PNG.

Steps to Reproduce

1. Call fig%savefig('output.dat') from any existing example (or run the snippet below).
2. Inspect the resulting file – the PNG backend is still active.

program repro
    use fortplot_utils, only: get_backend_from_filename
    implicit none

    print *, trim(get_backend_from_filename('output.dat'))
end program repro

The program above prints png, confirming the backend detection does not route .dat to the ASCII backend.

Expected Behavior

.dat should select the ASCII backend (same as .txt) so users get the text-friendly output documented in src/documentation/fortplot_documentation.f90.

Additional Context

The docs list .dat among the recognized media extensions, and several verification helpers emit .dat files in test/output/. Restoring ASCII routing keeps the CLI consistent with the documentation.