Use JSObject instead of JSFunction in Bun.plugin

[Jarred-Sumner]

What does this PR do?

Use JSObject instead of JSFunction in Bun.plugin

Hopefully will fix this:

Bun v1.1.42 ([50eec00](https://github.com/oven-sh/bun/tree/50eec0025b482ad7b340c82f64749903b2fad4b8)) on macos aarch64 [AutoCommand]

Segmentation fault at address 0x00000000

1 unknown/js code
Bun::getIfPropertyExistsPrototypePollutionMitigationUnsafe
Bun::getIfPropertyExistsPrototypePollutionMitigationUnsafe
JSC__JSValue__fastGet
src.bundler.PluginRunner.onResolveJSC
src.bun.js.javascript.VirtualMachine.resolveMaybeNeedsTrailingSlash__anon_124181
Zig__GlobalObject__resolve
Zig::GlobalObject::moduleLoaderResolve
JSC::JSModuleLoader::resolve
JSC::moduleLoaderResolve

Features: Bun.stdin, jsc, spawn, transpiler_cache

How did you verify your code works?

[robobun]

^{Updated 11:31 AM PT - Dec 23rd, 2024}

❌ @Jarred-Sumner, your commit 03d71af has 1 failures in #8462:

• test/js/node/test/parallel/test-zlib-dictionary.js - code 1 on 🐧 3.20 x64-baseline

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🧪   try this PR locally:

bunx bun-pr 15968