add --outbase option

[nitsky]

Currently, esbuild uses the lowest common ancestor of the entrypoints to determine where to place each entrypoint's output file. This is an excellent default, but is not ideal in some situations. Take for example an app with a folder structure similar to Next.js, with js files at pages/a/b/c.js and pages/a/b/d.js. These two files correspond to the paths /a/b/c and /a/b/d. Ideally, esbuild would emit out/a/b/c.js and out/a/b/d.js. However, esbuild identifies pages/a/b as the lowest common ancestor and emits out/c.js and out/d.js. This PR introduces a --outbase argument to the cli that allows the user to choose which path to base entrypoint output paths on. With this change, running esbuild with --outbase=pages results in the desired behavior.

[evanw]

This addition makes sense to me 👍