FASTA: User should be able to add, save and open FASTA/Sequence files with "X" symbol Peptides.

[NadezhdaPeskun]

Steps to Reproduce

1. Macro
2. Peptides >> Add 2-3 Peptides from X block
3. Save as Fasta and Sequence files (attachment file)
   
4. Try to open these files again as Fasta/Sequence - Peptides

Actual behavior.
User can add "X" Peptides and save to FASTA/Sequence but cannot open these files again.

Expected behavior
User should be able to add, save and open FASTA/Sequence files with "X" symbol Peptides.

Attachment
ketcher (XX fasta).zip
ketcher (XX seq).zip

Environment details:
Ketcher Version 2.20.0-rc.2 Build at 2024-03-27; 08:30:51
Indigo Toolkit Version 1.19.0-rc.2.0-g6c0e3fecf-x86_64-linux-gnu-11.2.1
Win10
Chrome Version 122.0.6261.129 (Official Build) (64-bit)

Related issues:

• Import and export fasta format for RNA, DNA and PEPTIDES Indigo#1755
• Autotests: Macro: UI for Open/Save As FASTA/Sequence #4280
• Fasta: All Peptides should be saved to FASTA format Indigo#1822