require space after `>` for quote

currently >foo at start-of-line is parsed as a quote; that's not what most Markdown parsers do, they require a space (> foo)

add tests, change the code, look out for weird interactions

(maybe only require a space before non-kanji/hanzi?)

thanks to @cheri@chan.thebeans.zip for reporting (see also https://wetdry.world/@freeplay/112016006454454360 )