add --force-tree-shaking flag

[yyx990803]

closes #1610

Please see committed changelog entry for details.

[yyx990803]

Some additional details - I considered some other options without adding a new flag:

1. Add another enumerated value to --tree-shake, like --tree-shake=force. However, it would be ambiguous how it interacts with --tree-shake=ignore-annotations. Ideally, we want to keep the ability to force treeshaking but still ignore annotations.

2. Making it an implicit behavior when --minify is enabled, as in most cases minification implies removing unused code. However, this would technically be a breaking change so I tried to avoid it.

I'm open to any form of API adjustments as long as it allows us to minify a single file using transform with tree-shaking enabled.

[yyx990803]

I just realized the current --minify behavior is already non-concatenation safe when processing an ESM input file when a target format is specified. This also applies when it implicitly detects ESM syntax:

Given this input:

function foo() {}
export { foo }

Result of running esbuild --minify:

function n(){}export{n as bar};

Notice foo has been renamed, which will break if another concatenated file expects to call foo().

That is to say, esbuild already assumes isolated scope when it is in ModeConvertFormat. With this in mind, I believe the correct behavior would be auto enabling tree-shaking when (1) --minify is enabled and (2) running in ModeConvertFormat. Since --minify was concatenation-unsafe to begin with, this should not be considered a breaking change.

We can also add a separate --minify-unused flag if necessary.

[yyx990803]

Closing in favor of #1617