Add a functional `write()` method to SpectrumList

The desired output of `SpectrumList.write()` would be a multiextension FITS file, where each extension contains the information from a single Spectrum1D object. The primary HDU could contain information about the number of included spectra, and possibly any common FITS keywords among the constituent Spectrum1D objects' metadata.

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The current error message when one attempts to use the `write()` method is:
```
astropy.io.registry.base.IORegistryError: No writer defined for format 'tabular-fits' and class 'SpectrumList'.

The available formats are:

Format Read Write Auto-identify
------ ---- ----- -------------
```

Start by tracing the SpectrumList.write() entry point and the tabular-fits registry error, then inspect how Spectrum1D objects expose their information for I/O. Define the primary-HDU metadata and per-spectrum extension contents from the issue, and verify that writing a SpectrumList produces a readable multiextension FITS file.

Written by the indexing model from the issue text.