feat: allow hash keys to be resolved as template attributes

[papilip]

Summary

Allow Hash keys to be used as template attributes, so that {{ my_hash.my_key }} resolves to the value associated with my_key.

Before: Only methods listed in template_attributes (like empty?, size, keys) were accessible. Hash keys were silently ignored, resulting in empty values with no error — a common pitfall.

After: Hash keys are resolved first, with a fallback to the standard template_attributes methods.

Motivation

When passing a Hash(String, String) to a template context:

render "my_template.html", context: {
  item: {"type" => "order", "amount" => "60.00"},
}

Users expect {{ item.type }} to output order. Instead, it silently outputs nothing because Hash only exposes methods like empty? and size to the template engine.

This is particularly confusing because:

• No error is raised — values are just empty
• NamedTuple works as expected via Object::Auto
• The fix is non-obvious (requires creating a custom struct with Object::Auto)

Changes

• src/marten/template/ext/hash.cr: Override resolve_template_attribute to check has_key?(key) before falling back to the macro-generated method
• spec/marten/template/ext/hash_spec.cr: 4 new tests covering key access, fallback to standard attributes, and error on unknown keys

Test plan

• Existing hash spec tests still pass (9 tests)
• New tests for key access (4 tests)
• size attribute still works when a key named size does not exist (fallback)
• UnknownVariable is raised for non-existent keys that are not standard attributes

[ellmetha]

Unfortunately, this change has serious limitations. If a hash contains a key like size, then it will be impossible to get the size from the hash when calling {{ my_hash.size }}. Similarly, any other method specified in #template_attributes will only work as long as a key with the same name is not part of the hash.

I think that we should give access to hash methods with priority over keys. That said, I don't want to allow hash key lookups if there is not a built-in way to actually do key lookups for keys such as "size".

The real way forward would be to add both support for dotted access and bracket access (eg. {{ my_hash["size"] }}).