feat(wintertc): migrate atob/btoa from boa_runtime

[KaustubhOG]

This Pull Request is part of the TC55 migration tracked in #4988, and builds on the skeleton crate from #5105.

It moves the base64 module (atob and btoa) from boa_runtime into the dedicated boa_wintertc crate as the first migration step toward hosting the WinterTC TC55 Minimum Common Web API in its own crate.

It changes the following:

• Moves the base64 module (mod.rs and tests.rs) into boa_wintertc. The implementation is unchanged; the only difference from the boa_runtime version is a short TC55 status note added to the module documentation.
• Ports the test harness (TestAction and run_test_actions_with) into boa_wintertc so the migrated unit tests run unchanged. This harness will be reused by later migrations.
• Adds dependencies to core/wintertc/Cargo.toml: the base64 dependency, plus indoc, textwrap, and futures-lite as dev dependencies.

Line counts (before and after, in both locations):

File	boa_runtime (source)	boa_wintertc before	boa_wintertc after
base64/mod.rs	82 (unchanged)	23 (stub)	86
base64/tests.rs	154 (unchanged)	0 (did not exist)	154

The 4 extra lines in mod.rs are the TC55 status doc note; the code itself is identical. tests.rs is byte for byte the same.

Approach: boa_runtime keeps its own base64 module for now, so there is no behaviour change for existing users and no new cross-crate dependency is introduced in this PR. Wiring boa_runtime to re-export from boa_wintertc (and removing the duplicate) is intended as a follow up, once the dependency direction between the two crates is agreed.

Testing:

• cargo test -p boa_wintertc: all six base64 unit tests pass, plus the crate doctest.
• cargo clippy -p boa_wintertc --all-features --all-targets: clean, no warnings.
• cargo fmt -p boa_wintertc -- --check: clean.
• cargo test -p boa_runtime base64: still passes (source untouched).

Out of scope:

• WPT is not run here: the WPT runner currently targets boa_runtime, so it does not yet exercise the boa_wintertc copy. That is covered automatically once the re-export follow up lands.
• Test262 does not apply, as atob and btoa are Web APIs rather than ECMAScript.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	51,072	51,072	0
Ignored	1,482	1,482	0
Failed	571	571	0
Panics	0	0	0
Conformance	96.14%	96.14%	0.00%

Tested main commit: 8a1e8fe07f626f7a067afc2c9885d5d87de4bb5d
Tested PR commit: b6a01d5620533055b0ff48ffebc0788fc4dc2c7a
Compare commits: 8a1e8fe...b6a01d5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.32%. Comparing base (6ddc2b4) to head (b6a01d5).
⚠️ Report is 985 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5418       +/-   ##
===========================================
+ Coverage   47.24%   60.32%   +13.07%     
===========================================
  Files         476      567       +91     
  Lines       46892    63163    +16271     
===========================================
+ Hits        22154    38102    +15948     
- Misses      24738    25061      +323     

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