Refactor `boa_string` dependency and GC trait implementations for release

[shruti2522]

Description:
Currently, oscars depends on the boa_string crate via a git dependency to provide optional Trace and Finalize implementations for boa_string types:

# oscars/Cargo.toml
boa_string = { git = "https://github.com/boa-dev/boa.git", branch = "main", optional = true }

While this works for our experimental integration, it poses a blocker for publishing oscars to crates.io in the future. Crates published to crates.io cannot depend on unreleased git repositories.

Furthermore, having oscars depend downstream on boa's internal crates creates tight coupling and architectural inversion. To resolve this correctly for a release, we need to evaluate the following long-term solutions:

1. Move oscars into the boa workspace:
   Bring the oscars garbage collector directly into the boa-dev/boa monorepo. This eliminates external versioning conflicts and allows internal crates to depend on each other seamlessly.

2. Move trait implementations into boa_string:
   Instead of oscars depending on boa_string to implement Trace/Finalize for it, boa_string should depend on the oscars crate (or an oscars-core crate that provides the traits) and implement the GC traits locally.

Related PR: (#96 (comment))

[shruti2522]

Currently resolved by PR #98.

The boa_string dependency has been completely removed from oscars/Cargo.toml to break the cyclic dependency. The Trace and Finalize implementations for JsString have now been moved directly into the boa_string crate over in boa-dev/boa#5460.