Stronger selector typing. Resolves #29.

[nsaunders]

Description

1. The CustomAttribute type and att constructor function have been removed in favor of the AttrName type from web-html.
2. The &. operator (byClass function) no longer accepts a string argument. Instead, it requires a ClassName.
3. The &# operator (byId function) no longer accepts a string argument. Instead, it requires a value of the newly-added Identifier type.

Design considerations

• These changes create a consistent pattern for creating and using identifier, class name, and attribute name values, also in alignment with keyframe names (see Drop keyframesName function, exposing KeyframesName constructor instead. #34).
• The ClassName and AttrName types originated in Halogen and were ported to web-html. Therefore, I would consider these "widely-recognized" types and their use (compared to bare strings) a best practice.
• The removal of the att and keyframesName functions follows a change made in Halogen a long time ago related to the ClassName type. (See Drop keyframesName function, exposing KeyframesName constructor instead. #34 for further discussion.)
• The purescript-tecton-halogen package will no longer require its own &. operator: One less import and one less potentially confusing thing.

Future plans

Advocate for Identifier to be added to a purescript-web library so Tecton doesn't have to own it. See purescript-web/purescript-web-dom#57.

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.