Support esbuild plugin onStart, onEnd, and onDispose callbacks

[paperclover]

Currently, we do not implement these three callbacks, but instead throw an error pointing to this issue.

They are very trivial to implement in a non-reloading setting, and mostly simple in a watch/hmr build. Usage is documented here.

Keep this issue open until:

• onStart
• onEnd
• onDispose
• these hooks within rebuilds (bake dev server tracking issue #14324)
• resolve(), prob open a separate issue

[Enalmada]

I am creating an npm module that requires 'use client'; directive to remain at the top of the file after build. bun is stripping it out right now.

It looks like esbuild is recommending the banner feature for equivalent functionality. I believe I could make a bun plugin to put 'use client' back this but need onEnd supported. Thank you for implementing when it makes sense.

[HuakunShen]

What's the status of this?
Need the onEnd callback.

[BjornTheProgrammer]

Notably, onEnd could also help fix #15033 by creating a plugin that outputs the relevant information to the end user, as onEnd should have access to metafiles.

[mProjectsCode]

onEnd missing currently blocks esbuild-svelte 0.9.0 from working.

# Unhandled error between tests
-------------------------------
263 |       });
264 |       build.onLoad({ filter: FAKE_CSS_FILTER, namespace: "fakecss" }, ({ path }) => {
265 |         const css = cssCode.get(path);
266 |         return css ? { contents: css, loader: "css", resolveDir: dirname(path) } : null;
267 |       });
268 |       build.onEnd(() => {
                  ^
TypeError: build.onEnd is not a function. (In 'build.onEnd(() => {
          if (!options)
            options = {};
          if (options.cache === void 0)
            options.cache = !0;
        })', 'build.onEnd' is undefined)
      at setup (***/node_modules/esbuild-svelte/dist/index.js:268:13)
      at ***/tests/__preload__/svelteLoader.ts:5:1

[BjornTheProgrammer]

I forgot to put it here, but I'm actively developing the onEnd hook. I can put up the branch if anyone else is interested in helping.