xmir-to-phi doesn't fail on invalid attribute names

[volodya-lombrozo]

When I run xmir-to-phi goal I successfully convert all xmir files into PHI expressions. However, recently we realised that PHI expressions we get have a mistake:

I-0 ↦ Φ.org.eolang.param,

AFAIK, in PHI, all attribute labels must start with a lowercase letter.

You can read more about it here.
Expected behaviour: xmir-to-phi throws an error that is saying about the wrong naming.

Here is my configuration:

<plugin>
  <groupId>org.eolang</groupId>
  <artifactId>eo-maven-plugin</artifactId>
  <version>0.39.0</version>
  <executions>
    <execution>
      <id>convert-xmir-to-phi</id>
      <phase>process-classes</phase>
      <goals>
        <goal>xmir-to-phi</goal>
      </goals>
      <configuration>
        <phiInputDir>${opeo.modified}</phiInputDir>
        <phiOutputDir>${eo.phi.output}</phiOutputDir>
      </configuration>
    </execution>
  </executions>
</plugin>

[volodya-lombrozo]

@maxonfjvipon Please, have a look

[yegor256]

@maxonfjvipon would be great to add a check into xmir-to-phi, to fail faster