add `genSrcs` keyword field to PathConfig

[jurgenvinju]

Is your feature request related to a problem? Please describe.

• The Rascal compiler needs a place to put the generated Java files before they are compiled by the Java compiler
• The bin field is not the right place to put it, since that is the root of generated .class files and resources such as .constants and .tpl

Describe the solution you'd like

• rascal-core already uses genSrcs internally as an extension to PathConfig, so let's keep that
• apply to util::Reflective where PathConfig is defined and add utility functions parallel to the support for bin
• PathConfig.java, RascalManifest, ShellEvaluatorFactory and such need to be extended to produce proper bindings for genSrcs
• This has impact in other projects, such as usethesource/rascal-language-servers, usethesource/rascal-maven-plugin and usethesource/rascal-eclipse