Set CWD programmatically

[ggoodman]

Description

Support passing in a cwd option to Service#build().

Use-case

Currently, much of esbuild's path resolution and output path generation appears to depend on the working directory. Using esbuild as a component in a larger system means careful management of the working directory in order to prevent any conflicts with other components that also rely on cwd. I anecdotally observed that cwd also appears to be determined at the time the service is spawned and remains bound to that for its lifetime. That simplified working directory management, but I think esbuild could allow this to be explicit.

[evanw]

Yes I agree. I'm planning to do this in a breaking change. This will also help allow sharing the underlying service between builds with different working directories. Right now they spawn separate processes.