minify: provide an option to enable treeshaking in single-file transform

[yyx990803]

In 4d32f7c it was explained that treeshaking is disabled in single file transform due to esbuild not seeing the entire bundle. However, there are use cases where:

1. We know the file is a single ES module that won't be concatenated with other files;
2. We do NOT want to use build --bundle because we want to minify the file in isolation.
3. We also do NOT want to use --format iife because we don't want to change the format of the file.

A real world example is where Vite using esbuild as a minifier to minify ESM chunks produced by Rollup.

Currently there seems to be no way to force esbuild to enable top-level treeshaking when using the transform API - this creates some issues when some tools' codegen leaves unused declarations expecting them to be removed by minifiers.

I think it'd be nice to have an option to force enable this. Maybe just respect treeShaking: true when calling the transform API.

[yyx990803]

Actually this is why I've found a somewhat unpredictable 10%~20% difference compared to Terser when I tested using esbuild as the minifier for Vite in real world cases - in some cases it's Rollup's treeshaking failing, in some cases it's plugins leaving unused code expecting the minifier to remove them.

I think having this option would allow Vite to default to esbuild as the minifier.

[hyrious]

It is also possible to do what you said in bundle api with a plugin that externalizes everything except the entry point: #1576 (comment). But I do think it is valuable to do tree-shaking in transform too.

However, as I know vite uses rollup to build the dist, which does tree-shaking a lot (at least more than esbuild). Therefore I don't think applying esbuild on rollup's result could make dist smaller.

[yyx990803]

Our use case is a bit niche, since we rely on disabling Rollup's treeshaking for certain modules for module graph analysis in bundle generation phase, and instead rely on the minifier to remove those forced-included modules.

There are also some cases where Rollup bails out by default (e.g. try catch blocks) despite a conditional branch that will never execute post transform, causing all code used by that branch to be included.