Support for "scripts" in package manifest

[mogorman]

It would be nice to have the toolchains have the option to be built locally. This would make platformio more portable and sustainable in the future.

[ivankravets]

Allow assigning different scripts per events from PlatformIO Package Manager via "scripts" property. Example, https://docs.npmjs.com/misc/scripts

Having "postinstall" action will allow calling a build script where we can build a toolchain/tools from the source code.

[ivankravets]

Sorry that it took so many years :(