It's no longer necessary to strip shebangs from modules

[andreubotella]

Deno's module fetcher will strip a shebang if a module file starts with one. However, there is a stage-3 TC39 proposal to support shebangs (or "hashbangs") as part of JS syntax. In fact, this was implemented in V8 in 2018 and has shipped by default since V8 7.4. So this transformation can be removed.

This affects the output of Deno.emit() though. Although emit::bundle will skip any shebangs in the source files, and so deno bundle and Deno.emit() with the bundle parameter are safe, emit::to_file_map (which powers Deno.emit() without the bundle option) will output shebangs for the files that have them. This might be bad, depending on what kinds of environments the code generated by Deno.emit() will be used in. We should consider whether to strip the shebang from the outputs.

Another potential issue is positions in sourcemaps: since currently shebangs are stripped before the module is parsed, any sourcemaps generated by tsc or swc would be off by one line for modules with shebangs. I don't know if this error is corrected at all, but if so, the correction would have to be undone. Also, if shebangs are stripped from the Deno.emit() outputs, those sourcemaps would also need to be corrected to account for it.

[andreubotella]

Given #13021 and #13188 (comment), the sourcemap issues are probably not a blocker for this change.

Also, I noticed that, because the shebang is being stripped before the BOM, a file with both a BOM and a shebang will not have its shebang stripped, so shebangs can show up in the outputs to Deno.emit(). This is almost certainly unintentional, though, and we should consider whether that is expected.

I also realized that this shebang stripping is causing a correctness bug with JSON modules, since the shebang will be stripped when the file is fetched, regardless of the module type.