text-minimessage: Allow tags to be closed in one tag

[zml2008]

This simplifies the language for tags with no children.

[rymiel]

This doesn't implement #523, just adds the tokenization capabilities, right?
Is emitting lang and key tags as self-closing something we want? Maybe makes it imply those have children taking #523 into context

[rymiel]

disregard the first part of that comment, testIgnorableSelfClosable addresses that

[zml2008]

To support vanilla components properly, I made all of the standard tags able to have children, since all component types can have children normally. The forced-no children attribute on Inserting tags is really just for custom things -- so there should probably be an attribute on the serializer to generate those sort of tags without the closing slash.

The only situation where that's relevant in MM itself is probably the newline tag though, since that's theoretically serializable (though, due to compaction it often won't end up as a separate component).

[rymiel]

Today i learned those components can have children in vanilla, sounds messy