Change default merge strategy for markup.goldmark.extensions.typographer and others

[salim-b]

Since #10620 has been resolved, we can set per-language markup config like

[languages.de.markup.goldmark.extensions.typographer]
leftDoubleQuote = '«'   # default “
rightDoubleQuote = '»'  # default ”

Now one mustn't forget to also set

[languages.de.markup]
_merge = 'deep'

since otherwise other options from regular markup config aren't respected for lang de and Hugo falls back to built-in defaults.

Would be nice if _merge = 'deep' could become the default for languages.<LangPrefix>.markup.

[bep]

Would be nice if _merge = 'deep' could become the default for languages..markup.

I don't think we can/want to do it so coarse grained. I would hate for a random theme to set markup.goldmark.renderer.unsafe=true e.g.

I think what we need to do is:

• Make sure that all settings below languages gets the same default merge strategy as they would have gotten on the top level (I'm not sure that is the case today)
• Revise the current defaults and open up markup.goldmark.extensions.typographer and possibly others.