Ignore some files in npm package

[JLHwung]

Q	A
Fixed Issues?	Some files were unexpectedly published to npm
License	MIT

Ignore unused file in npm package. Here is an unpacked size comparison

package	before	after
generator	117 kB	116.5 kB
plugin-proposal-pipeline-operator	29.4 kB	8.3 kB
plugin-syntax-pipeline-operator	3.74 kB	3.0 kB
plugin-transform-dotall-regex	4.2 kB	2.9 kB
preset-typescript	6.34 kB	3.6 kB
standalone	3.69 MB	3.69 MB
env-standalone	4.33 MB	4.33 MB

[JLHwung]

I remove the src here because

1. It is untranspiled since day 1 of @babel/standalone and nobody ever complains about that
2. src is not intended to be used by end users