Add a simple frontend demo

[certik]

This libasr repository should depend on LFortran as a submodule for "upstream libasr", and contain a simple demo. A good idea is a frontend for Pascal or Oberon, which can be parsed with a simple recursive descent parser and single pass (the full grammar is short: https://en.wikipedia.org/wiki/Oberon-2#Syntax). So it will be just one file that does the parsing and semantics at the same time, and constructs ASR. Then in the main program we expose the various backends of libasr (C, Fortran, WASM, etc.).

We will maintain this as a template how to start new frontends.

Here is the full parser for Oberon-2: https://grok.com/share/bGVnYWN5_cf6f61b3-557f-49ed-b6ba-8e6551d7fbc1.