solved vulnerabilities, added types definition and updated readme

[jon2180]

Changelog

• replaced new Buffer(string) (DEPRECATED) with Buffer.from(string)
• replaced 'var' with 'let/const'
• updated dependencies to solve vulnerabilities
• migrated from 'readable-stream' to stream(node-core) and remove the dependency
• updated readme for gulp 4
• added types definition

[jon2180]

@lazd Whether obsolete code demonstrate for gulp 3 needs to be removed. I think the code is unnecessary and redundant.

[lazd]

@jon2180 we could throw it in another file, maybe README-gulp3.md?

[jon2180]

@jon2180 we could throw it in another file, maybe README-gulp3.md?

That sounds nice