prohibit inner inner objects with `@base` attributes if it can be merged with its parent

In EO 0.51.0 we introduced roll-bases.xsl that reduced amount of nested objects and merges their @base attributes if it's possible.
Let's make a lint that checks if all possible bases are merged.

Imagine we have two objects:

<o base="X">
  <o base="Y"/>
</o>

If they are merged we get <o base="YX"/>.

We can't merge them if:

1. X does not start with . (dot)
2. X starts with . and Y starts with .
3. Y has inner object Z that can't be merged with Y (apply all the rules for Y-Z pair)

<o base="X">
  <o base="Y">
    <o base="Z"/>
  </o>
</o>

4. Y has more than 1 any inner objects
5. Y has data (not empty text())
6. Y is an abstract object

For more details see the roll-bases.xsl itself

Here X and Y are just variables in common sense, not a specific bases. It can be .seq, $.seq or Q.org.eolang.seq or something similar under the X and Y.

The severity level is warning (temporary)