Add SaltXml exporter

[thomaskrause]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 84.42623% with 228 lines in your changes missing coverage. Please review.

Project coverage is 82.88%. Comparing base (ff957db) to head (f45c3cb).
Report is 61 commits behind head on main.

Files	Patch %	Lines
src/exporter/saltxml/document.rs	84.93%	6 Missing and 77 partials ⚠️
src/exporter/saltxml.rs	81.95%	18 Missing and 41 partials ⚠️
src/util.rs	73.33%	19 Missing and 13 partials ⚠️
src/importer/xlsx.rs	86.80%	12 Missing and 7 partials ⚠️
src/exporter/saltxml/corpus_structure.rs	79.76%	1 Missing and 16 partials ⚠️
src/importer/saltxml/document.rs	75.43%	7 Missing and 7 partials ⚠️
src/importer/saltxml.rs	77.77%	2 Missing ⚠️
src/manipulator/visualize.rs	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #292      +/-   ##
==========================================
+ Coverage   82.69%   82.88%   +0.19%     
==========================================
  Files          48       51       +3     
  Lines       17344    18580    +1236     
  Branches    17344    18580    +1236     
==========================================
+ Hits        14342    15400    +1058     
- Misses       1120     1162      +42     
- Partials     1882     2018     +136     

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