Fix(#182): correct types and type checks

[SukkaW]

The PR closes #182.
X-Ref: parcel-bundler/parcel#7906

PostHTML does supports node.content to be either a string or a string[] when stringify a PostHTML AST Tree into HTML, while PostHTML parser will only emit node.content as an array of string. So the new htmlnano module syntax (#128) just take it for granted that node.content will always be a string[].

However, some htmlnano modules (minifyJson and minifySvg) will transform node.content into a string instead of a string[], so when other modules (which expects node.content to be a string[]) process the new node.content later on, htmlnano will crash.

This PR fixes the issue by:

• Make sure minifyJson, minifySvg, and other modules will always return string[].
• node.content will always be cast into an array before passed into onContent.
• The result from onContent will always be cast into an array.
• Add additional type check when applying new syntax modules.

cc @maltsev @AndrewKvalheim @mischnic @arneke

[netlify]

‼️ Deploy request for htmlnano rejected.

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[maltsev]

Thanks! I'll release a new version this evening.