[Rpm-maint] [rpm-software-management/rpm] Add substring macro to expose lua's string.sub function (PR #2179)

[Panu Matilainen]

Besides using arg[n] instead of rpm.expand() on the args, checking for the arguments is counter-productive: the Lua-level function checks this by itself, does a  better job of it and the third argument is actually optional (see https://www.lua.org/manual/5.4/manual.html#pdf-string.sub)

So it all comes down to just this, which (AFAICS) can do everything the Lua version can, including handle negative numbers and two-argument form:
```
%sub(-) %{lua:return string.sub(arg[1], arg[2], arg[3])}
```

````
$ rpm --define "commit 2bc745f2fde028e09f663c7967353e8b6aacdbf1" --eval "%sub %{commit} 1 7"
2bc745f
$ rpm --define "commit 2bc745f2fde028e09f663c7967353e8b6aacdbf1" --eval "%sub %{commit} -5"
cdbf1
$ rpm --define "commit 2bc745f2fde028e09f663c7967353e8b6aacdbf1" --eval "%sub %{commit} 1 -7"
2bc745f2fde028e09f663c7967353e8b6a
```

And like in #1764, it's not a question of whether this is useful or not (it quite obviously is), but whether we shouldn't just wrap the entire Lua string library this way and be done with it.

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