Replace file-based esbuild config with consuming Jest's transformer options object

[nonAlgebraic]

As described in #2 , this PR replaces the existing logic of grabbing esbuild build options from a file named jest.esbuild with grabbing them from transformer options provided in the Jest config file. This removes the need to create such a file in the first place, and brings this transformer into better alignment with best practices for Jest transformers.

[AkifumiSato]

Thanks! Please also correct the README docs.