gen: reduce hardcoded JS for some compiler-magic features

[serkonda7]

Describe the feature

As of writing. the following places in jsgen have lot's of JS hardcoded in strings:

• for_in_loop
• for_in_map
• assert_stmt
• gen_test_main
• The whole file lib/bait/gen/js/str_methods.bt

This is prone to misspellings and errors if things are moved, renamed, etc.

Proposed Solution

The JS in strings should where possible be removed.
Instead in the parser or checker stage manipulate the AST and insert exprs or stmts that will lead to the generation of equivalent code utilizing the normal jsgen methods.

Additional Information

For example in the parser script_mode_main artificially inserts a FunDecl.