Rename Identifier type to ElementId.

[nsaunders]

Description

Following up on #35, this renames the newly added Identifier type to ElementId.

Design considerations

This change follows a discussion in purescript-web/purescript-web-dom#57 where we decided on ElementId as the name for this type which I proposed should be added to that library.

Future plans

My hope is for web-dom to own this type eventually, at which point Tecton can just re-export it similar to ClassName and AttrName.

References

N/A

Code change checklist

• Any new or updated functionality includes corresponding unit test coverage.
• I have verified code formatting, run the unit tests, and checked for any changes in the examples.
• I have added an entry to the Unreleased section of the CHANGELOG.