Allow user to configure locations of generated directories

[petervdonovan]

This RFC competes with #7. It is much more complex than #7 and has a wider scope. It is incompatible with #7 and would subsume #7 if it is merged.

The reasons for opening this RFC now are that:

• A serious UX problem (lf-lang/lingua-franca#2203) was recently reported, and it is preferable that we address it soon.
• The obvious solutions to this problem generally involve breaking changes, so it is desirable to solve it earlier rather than later.
• Solutions to this problem have the potential to affect the location of generated executables, which certainly will affect the correctness of the VS Code extension. More generally, changes in the locations of generated files can affect the extension depending on features that we may implement, such as interaction with third-party language servers. We strongly prefer for the VS Code extension to work properly because its behavior affects how our project is perceived by users; therefore, the desired behavior should be nailed down with minimal ambiguity.

For these reasons it is desirable for comments on this RFC to be received before the end of next week on Friday March 8 as a soft deadline. This tentative deadline can be adjusted based on feedback.

Rendered