fix(aliases): wikilink resolution for aliases (#1681)

With markdownLinkResolution: "shortest", aka "+/- how Obsidian does it"
and given pages A and nested/B which has an alias Z, if you try to link
from A using [[Z]] it wouldn't work and get 404.

This is caused by alias slugs (nested/Z in this case, emitted by
AliasRedirects) not being present in the `allSlugs` list which is used
by the link transformer.

The fix is to compute the alias slugs in the frontmatter transformer
and add them to `allSlugs` there.
Also we store them in file data to avoid recomputing them when emitting
alias redirect pages.

Fixes #904

Note: given how currently the markdown/html transformers are ordered
this doesn't really work.

Given pages A and nested/B which has an alias Z, here's the order which
currently happens:

md-transformers(A) => html-transformers(A) =>
md-transformers(B) => html-transformers(B)

Since the nested/Z slug will get added when md-transformers(B) are run,
but the slugs are used by html-transformers(A) when resolving it's
links - the link [[Z]] in A will still 404

A fix for this is to split the parser into two stages - first apply the
md-transformers to all files, and only then apply html-transformers to
all files.

I did just that in a different commit, which is needed for this one to
work correctly.