feat(jstz_engine): add support for creating, entering and exiting Realms

[johnyob]

Context

Adds an interface for Realms. A realm represents a distinct execution environment for JavaScript code,
encapsulating global objects, intrinsic objects, and a separate environment for executing scripts and modules.

Description

This PR adds the ability to:

• Create new realms (which currently, by default, creates a new compartment),
• Enter new realms using Context::enter_realm
• Leave the realm once the Context is dropped.

Manually testing the PR

cargo nextest run --package jstz_engine

[codecov]

Codecov Report

Attention: Patch coverage is 76.19048% with 25 lines in your changes missing coverage. Please review.

Project coverage is 47.21%. Comparing base (312043f) to head (cff69fb).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/jstz_engine/src/realm.rs	66.66%	13 Missing and 1 partial ⚠️
crates/jstz_engine/src/context.rs	82.53%	10 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
crates/jstz_engine/src/lib.rs	100.00% <ø> (ø)
crates/jstz_engine/src/context.rs	73.21% <82.53%> (+34.57%)	⬆️
crates/jstz_engine/src/realm.rs	66.66% <66.66%> (ø)

... and 1 file with indirect coverage changes

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