implement first additional input language

[soc]

Motivation

Over the last decades it has become clear that providing access to language-external schemas and types is important.

But the techniques to do so, like code generation, compiler plugins (e. g. Manifold), macros, type providers and direct embedding have each shown their weaknesses.

Instead of going down this path again, make these schema definitions first-class citizens, by accepting them as valid source files.

This means that *.properties, *.xsd, *.jssd, etc. shall be treated similar to *.core files, i. e. they are parsed according to their grammar, an AST is constructed and typed, and code is emitted.

Goal

Implement support for *.properties files as first-class citizens in Core.

Tasks

• make Core pick up *.properties files
• lex and parse *.properties files
• build AST for *.properties files
• typecheck *.properties files