feat(grammar): support feature gates on nested items

[avasich]

Description

This PR fixes a parse error where nested feature gates were not recognized by the grammar, and does a minor refactor to align item bodies closer to the spec.

1. Nested Feature Gates

Currently, tree-sitter parse fails on valid code with gates inside resources:

interface poll {
    resource pollable {
        @since(version = 0.2.0)
        ready: func() -> bool;
    }
}

This patch adds optional($._gate) to _resource_body.

Furthermore, I added optional($._gate) to _record_fields, _flags_fields, _variant_cases, and _enum_cases. Given the precedent of prioritizing practicality over strict spec adherence (e.g. #30), supporting these seems appropriate.

2. Grammar Shape Refactoring

Refactored _world_body and _interface_body to hoist the gate parsing out of the specific item choices. This aligns the grammar structure with the spec:

world-items ::= gate world-definition

matching the existing implementation for _resource_body.

[mkatychev]

Hi @avasich thanks for the submission. I will get to this later this week, was preoccupied with other projects.