Add builtin keyword helpers (fn, hash, array, and, or, not, eq, neq, lt, lte, gt, gte)

[NullVoxPopuli-ai-agent]

Summary

Building on the foundation from #21068 (on as keyword), this PR adds all remaining helpers as strict-mode keywords per their respective RFCs:

• fn (RFC #998), hash (RFC #999), array (RFC #1000) — already existed as helpers, now registered as keywords
• and, or, not (RFC #562) — new helper implementations + keyword registration
• lt, lte, gt, gte (RFC #561) — new helper implementations + keyword registration
• eq, neq (RFC #560) — new helper implementations + keyword registration

What this means

In strict-mode templates (.gjs/.gts/<template>), these helpers are now available without importing them:

<template>
  {{! No imports needed for any of these }}
  <button {{on "click" (fn this.doStuff "arg")}}>Click</button>
  {{#each (array "a" "b" "c") as |item|}}{{item}}{{/each}}
  {{#let (hash name="Ember") as |obj|}}{{obj.name}}{{/let}}
  {{if (eq this.status "active") "Active" "Inactive"}}
  {{if (and this.isReady (not this.isLoading)) "Go" "Wait"}}
  {{if (gt this.count 10) "Many" "Few"}}
</template>

All keywords can still be shadowed by local scope definitions, maintaining backwards compatibility.

Implementation

• New helpers (and, or, not, eq, neq, lt, lte, gt, gte) implemented as Glimmer internal helpers in @glimmer/runtime
• All helpers exported from @ember/helper
• auto-import-builtins AST plugin extended to handle MustacheStatement and SubExpression nodes (in addition to ElementModifierStatement for on)
• Strict mode only — no changes to loose mode (hbs) templates

Not included

• element (RFC #389) — this involves contextual components and needs separate implementation

Test plan

• Integration tests for each keyword helper (explicit scope, keyword-only scope, runtime compiler)
• Tests for shadowing behavior
• Smoke tests for keyword usage in a real Ember app
• All existing tests pass (8794 tests, 0 failures)
• TypeScript types pass
• Lints pass (ESLint, Prettier, docs coverage)

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