gh-84461: Improve WebAssembly in-browser demo (#91879)

* Buffer standard input line-by-line

* Add non-root .editorconfig for JS & HTML indent

* Add support for clearing REPL with CTRL+L

* Support unicode in stdout and stderr

* Remove \r\n normalization

* Note that local .editorconfig file extends root

* Only normalize lone \r characters (convert to \n)

* Skip non-printable characters in buffered input

* Fix Safari bug (regex lookbehind not supported)

Co-authored-by: Christian Heimes <christian@python.org>