Macro: Obtain sequence for modified chains

**Background**
Users need to be able to see the unmodified, natural sequence of a modified nucleotide. That way they can search for the unmodified sequence to find all the modified forms.
The same rule could be applied also for peptide sequences.

**Requirements**
1. User should be able to save modified chain as sequence made up from natural analogs of monomers.
2. For RNA: phosphates and sugars that are not a part of nucleotide or nucleoside are not exported.
3. Chems are not exported.
Examples:
- Peptide

Sequence: ARGFCKAEDA
- RNA

Sequence: TCA

No files, tests, or entry points are named in the issue. Start by locating the code that exports modified peptide and RNA chains as sequences, then trace how monomers, phosphates, sugars, and chems are represented. Done means natural analog sequences match the peptide and RNA examples while excluding the specified non-exported components.

Written by the indexing model from the issue text.