Moved get helper deduplication behind a labs flag (#28267)

ref https://linear.app/ghost/issue/PLA-55

Per-request `{{#get}}` helper deduplication (shipped in #26487) was
gated by the `optimization:getHelper:deduplication` config setting. This
moves the gate to a labs flag so it can be toggled per-site through labs
config instead of needing a separate config setting — which is what we
need to roll it out to sites individually.