Add temporary rules for @content and @import at rules to JS AST

[sanderlenaerts]

Hey there,

It seems the sass-parser currently crashes upon reaching an @content or @import statement.
This currently throws visitor.visitContentRule$1 is not a function as visitContentRule and vistiImportFile are not part of

dart-sass/pkg/sass-parser/lib/src/statement/index.ts

Line 201 in 89a5394

visitAtRule: inner => new GenericAtRule(undefined, inner),

Until full-fletched support is added for these at rules, can we add them and let them use the GenericAtRule?