Tagstack is always appended to, behaves different to native tags

Currently ale doesn't replicate Vim's native tag behavior when using `:ALEGoToDefinition`. The way that the tagstack is updated is different.

Natively, `CTRL-T` will go up in the tagstack, decrementing the "current index". A subsequent `CTRL-]` will push to the tagstack at the position of the current index, replacing everything that came after this index.

Ale, however, always appends at the end of the tagstack, ignoring the current index. This breaks `CTRL-T` because it will jump to the last definition that had been jumped to chronologically, rather than the one that was previously "current".

Unfortunately `settagstack` can only replace or append to the tagstack, so we have to manually replicate the behavior. (I've just opened https://github.com/vim/vim/issues/5405 to suggest to provide it natively.)