Fix XYZ syntax highlighting contexts

[mveril]

Summary

• Model XYZ highlighting as explicit main, comment, and atoms contexts.
• Keep the second XYZ line entirely in a comment scope so numbers in method names, basis sets, and URLs are not highlighted as coordinates.
• Restrict coordinate highlighting to complete atom records.
• Support concatenated XYZ frames by switching back to the comment context when a new atom-count line is encountered.
• Use variable.other.atom.xyz for atom symbols instead of entity.name.atom.xyz.

Before

24
Anthracene, B3PW91/6-31G(d,p) geometry, https://...
C  -6.199447  -0.542799  -0.014381

The generic numeric rule also highlighted numbers in the free-form comment line.

After

24                                      # atom count
Anthracene, B3PW91/6-31G(d,p) ...       # comment
C  -6.199447  -0.542799  -0.014381      # atom + coordinates

Only atom counts and coordinates receive numeric scopes, while the XYZ comment remains a single comment scope.

Verification

• Compared the branch against main; only assets/syntaxes/XYZ.sublime-syntax is changed.
• CI will rebuild the embedded bat syntax assets and run the repository checks.