feat: enable Jekyll-compatible dot notation in assign tags

[osteele]

Summary

• Updates the liquid template engine from v1.6.0 to v1.8.0
• Enables Jekyll extensions in the liquid engine, including support for dot notation in assign tags (e.g., {% assign my_obj.prop = "value" %})
• Updates CHANGELOG and Contributors

Closes #60

Details

The liquid v1.8.0 release adds EnableJekyllExtensions() which supports dot notation on the left-hand side of assign statements. This is now enabled by default in gojekyll's liquid engine.

Note: Ruby Jekyll does not actually support {% assign page.canonical_url = page.url %} (assigning to the page drop's properties). This works for custom variables but the page drop remains read-only, matching Jekyll's behavior.

Test plan

• Added test page with dot notation assign to testdata site
• All existing tests pass
• Build succeeds