Fix bundler crash with onLoad plugins on copy-file loaders used on entrypoints

[zackradisic]

What does this PR do?

This fixes a crash in the bundler when:

• the entry point is a file with a type which should be copied by the bundler (such as .wasm or .css with experimentalCss: false)
• an onLoad plugin matches this file

Two problems occurred in this case:

• the entry point was not appending to additional_files
• onLoadAsync callback was incorrectly appending string to free_list

[robobun]

:x: @zackradisic, your commit 3ecff6d7bcb361739010d8cb4204b4da025563bc has 12 failures in #6338:

test/cli/install/bun-add.test.ts - 2 failing on 🍎 13 x64

test/cli/install/bun-add.test.ts - timeout on 🍎 14 x64

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 aarch64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64

test/js/web/fetch/fetch.tls.test.ts - 1 failing on 🍎 14 aarch64

test/js/web/fetch/fetch.tls.test.ts - 1 failing on 🍎 13 aarch64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 aarch64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64-baseline

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64

test/js/bun/http/serve.test.ts - 1 failing on 🍎 13 aarch64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 aarch64

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64-baseline

test/js/bun/http/serve.test.ts - segmentation fault on 🐧 3.20 x64

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/bun/ffi/cc.test.ts - 1 failing on 🐧 3.20 x64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 aarch64

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64-baseline

test/cli/install/bun-link.test.ts - 4 failing on 🐧 3.20 x64

test/js/node/test/parallel/http2-pipe.test.js - code 1 on 🍎 14 aarch64

test/cli/hot/hot.test.ts - timeout on 🪟 2019 x64-baseline

test/js/third_party/pino/pino.test.js - segmentation fault at address 0x21e67a6e858 on 🪟 2019 x64