feat: add OPT_ENSURE_ASCII functionality, with tests

[DeflateAwning]

Changes

• Added orjson.OPT_ENSURE_ASCII functionality, which mirrors the json.dumps(..., ensure_ascii=True) implementation
• Added a complete grid search test which covers a variety of test objects with all combinations of OPT_ENSURE_ASCII, OPT_INDENT_2, and OPT_SORT_KEYS enabled, comparing back to json.dumps(...) versions of those (see test/test_dumps_indent_ascii.py file).
• Updated documentation in README to report new functionality.

Notes

• Note: Platform-specific optimized implementations of the new format_escaped_str_ascii function are not yet implemented. Currently, only a generic implementation is written. While optimizing this function is certainly possible, creating this MVP is what I set out to do first; I'd suggest that it is more than acceptable to merge in its current state.
• This is related to Issue Feature Request: Add an OPT_ENSURE_ASCII option value #580. I was motivated by JSON-encoding values in such a way that they can be used as values in AWS S3 Metadata values (which only support ASCII characters).

Remaining Actions

• Update change log. I have not updated the change log yet.
• I'd recommend a squash-merge into master. Let me know if you'd prefer that I clean up the commit history.

[DeflateAwning]

Just rebased to your changes in master from yesterday.

[DeflateAwning]

Just rebased again. Would really appeciate if you could look at this PR for any feedback/blockers!