feat(jstz_engine): add support for rooting GC values

[johnyob]

Context

See gc/root.rs for context.

Description

See gc/root.rs for described solution.

Manually testing the PR

cargo nextest run -p jstz_engine

[codecov]

Codecov Report

Attention: Patch coverage is 61.96078% with 97 lines in your changes missing coverage. Please review.

Project coverage is 45.65%. Comparing base (f55488f) to head (6146f87).

Files with missing lines	Patch %	Lines
crates/jstz_engine/src/gc/root.rs	71.15%	45 Missing ⚠️
crates/jstz_engine/src/script.rs	25.00%	36 Missing ⚠️
crates/jstz_engine/src/realm.rs	33.33%	10 Missing ⚠️
crates/jstz_engine/src/context.rs	83.33%	6 Missing ⚠️

Files with missing lines	Coverage Δ
crates/jstz_engine/src/gc/ptr.rs	66.15% <ø> (+15.38%)	⬆️
crates/jstz_engine/src/context.rs	74.65% <83.33%> (+2.33%)	⬆️
crates/jstz_engine/src/realm.rs	60.00% <33.33%> (-6.67%)	⬇️
crates/jstz_engine/src/script.rs	23.84% <25.00%> (+0.41%)	⬆️
crates/jstz_engine/src/gc/root.rs	71.15% <71.15%> (ø)

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