Refactor number of pages alias (fix #1090)

[andersonhc]

e.g. Fixes #0

Checklist:

• The GitHub pipeline is OK (green),
  meaning that both pylint (static code analyzer) and black (code formatter) are happy with the changes of this PR.

• A unit test is covering the code added / modified by this PR

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[gmischler]

Alias = "A different name for something"

That's not really what we're dealing with here, at all.

Yes, I know that .str_alias_nb_pages has had that name for like forever, but maybe we shouldn't propagate it to all the internal code dealing with text substitution.
In #1202 I tried to use terminology that describes better what really happens. I probably haven't found the absolute best wording, but it may help as a more reasonable starting point.

Using names that don't actually reflect the purpose of the named elements makes the code really confusing to read and hard to understand. I'm not a native english speaker, but I like to be as precise as possible about those things.

Other than that, I expect the result to "just work"... 😉

[andersonhc]

@Lucas-C @gmischler
This PR should be ready now.
I renamed the internal methods from "alias" to "text substitution" but didn't touch the ones that already existed (like alias_nb_pages) to keep backwards compatibility.

Lucas, I changed the text shaping enable/disable test you created recently because after this change the replaced number of pages is similar with text shaping on or off. I replaced by a text with ligatures that have internal and visual differences.