More changes to CI and subsequent fixes

[kaleidawave]

Aiming to fix: Issues with "no std" added in #374 around tests #399 and using String #389. Changed the CI to catch these issues. Modified examples and tests to run on different feature configurations.

Steps

• Add --no-default-features checks to CI
• Move StylesWriter to own module to avoid cfg repetition
• Move term.rs to term/mod.rs to avoid confusion
• Split emit to emit_to_string, emit_to_io_write and emit_to_write_style
• Add emit_to_write_color function for termcolor::WriteColor
• Update tests and get them working with --no-default-features
• Change start_context_lines back to before_label_lines and revert expected test
• Add CI "ecosystem" tests (warn if PR is tagged with breaking change)

[cwfitzgerald]

Thanks for working on this! Will fix our problem!