Warn on dot-notation assign instead of erroring

[osteele]

Summary

Change gojekyll to accept {% assign page.property = value %} syntax with a warning instead of failing the build. This improves compatibility with Ruby Jekyll, which silently accepts this syntax.

Background

Testing with Ruby Jekyll 4.4.1 confirms that {% assign page.custom_var = "from-assign" %} is parsed without error but is effectively a no-op:

Expression	Ruby Jekyll Output
{{ page.custom_var }} after {% assign page.custom_var = "from-assign" %}	from-frontmatter (assign did NOT override)
{{ page.new_var }} after {% assign page.new_var = "new-value" %}	(empty) (assign did NOT make it accessible)

The assign creates a variable under the literal key "page.custom_var" in the assigns hash, but {{ page.custom_var }} resolves via the page drop (page → .custom_var), a different lookup path.

Currently gojekyll errors: syntax error: dot notation in assign tag requires Jekyll extensions to be enabled. This breaks compatibility with Jekyll sites that use this syntax (see #60).

Proposed behavior

• Default mode: Accept the syntax, emit a warning: "{% assign page.custom_var = ... %} has no effect. In Jekyll, dot notation in assign creates an inaccessible variable."
• Future strict mode (--strict): Upgrade warnings to errors

Dependencies

Depends on osteele/liquid#144 — adding warnings support to liquid's render output. Once liquid can return warnings alongside rendered output, gojekyll can:

1. Collect warnings from all page renders
2. Print them to stderr
3. Optionally fail the build in strict mode

Related

• Feature request: object attribute assign #60 — original feature request
• feat: enable Jekyll-compatible dot notation in assign tags #120 — PR that enables EnableJekyllExtensions() (to be superseded)
• Add warnings support to render output liquid#144 — liquid engine warnings support