Rethink how feature checks are performed

[Omikhleia]

You can't make an omelette without breaking eggs

After struggling with making silex.sile v0.4 less aggressive and not replace SILE's internals globally... I realize that markdown.sile actually enforces the replacements indirectly...

This is due to the feature check it does, loading the resilient class (which enforces all of silex):

markdown.sile/packages/markdown/commands.lua

Lines 154 to 155 in 1a5411f

	local ok, ResilientBase = pcall(require, 'classes.resilient.base')
	self.isResilient = ok and self.class:is_a(ResilientBase)

So even processing a document using the bare "book" ends up loading the silex replacements at some point (and possibly in the course of a document processing, which then causes havoc)

We'd possibly need to do those checks differently, there's some wrong inversion of logic.

[Omikhleia]

Workaround in v1.5.2 so it works with silex v0.4, but general issue kept open -- it's perhaps time to trying preparing for SILE 0.15 and reconsider options.