Enable east_asian_line_breaks extension by default

[TomBener]

From issue #11241 (comment)

As for the default, I think it would be worth discussing whether to make this extension set by default. Perhaps open a question on Discussions here?

I agree that east_asian_line_breaks should be enabled by default.

It matches the natural behavior of Chinese and Japanese text (no spaces between characters) and aligns with how browsers handle soft line breaks in CJK text. For Korean, which uses spaces normally, this extension doesn’t interfere — it only suppresses unwanted spaces between full-width characters.

Enabling it by default would make Pandoc’s handling of CJK text more intuitive and reduce confusion for beginning CJK users, while those who prefer the old behavior can still disable it with -east_asian_line_breaks.

Additionally, it could make it possible for Quarto to support east_asian_line_breaks by default, see quarto-dev/quarto-cli#8520