feat(jstz_engine): add skeletons for Context and Compartment

[johnyob]

Context

A SpiderMonkey Runtime contains the state of the garbage collector. The collector partitions the JavaScript heap into compartments. Compartments have the invariant that no object in the compartment can contain pointers to another compartment.

A Context contains the state for a virtual machine that executes and manages JavaScript objects within a Runtime. It can compile and execute scripts, get and set object properties, call JavaScript functions, convert JavaScript data from one type to another, create objects, and so on.

Description

This PR adds the skeletons for the Compartment and Context types & traits.

We track the compartment that an object or context is in at the type-level using capability traits.

Manually testing the PR

cargo nextest run --package jstz_engine

[codecov]

Codecov Report

Attention: Patch coverage is 59.64912% with 23 lines in your changes missing coverage. Please review.

Project coverage is 45.83%. Comparing base (5fbe97d) to head (9c26683).

Files with missing lines	Patch %	Lines
crates/jstz_engine/src/context.rs	59.64%	23 Missing ⚠️

Files with missing lines	Coverage Δ
crates/jstz_engine/src/lib.rs	100.00% <ø> (+7.31%)	⬆️
crates/jstz_engine/src/context.rs	38.63% <59.64%> (ø)

... and 1 file with indirect coverage changes

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