fix: Fix cascading and fallback propagation of text styles

[luanpotter]

Description

Fix cascading and fallback propagation of text styles.

While using text nodes and flame_markdown, I noticed that basic propagating of styles was not working.

For example, if you create a standard document, it will have some margins by default. Even when setting it to zero, it was still overwritten by the default style.

I noticed that there was no consistency in the methods copyWith and merge of which parameter/receiver should be the master and which should be the fallback. Both the implementations and the callers would use it either way indiscriminately.

This consolidates the definition and adds comments to the methods enforcing the priority order.

In order to fully test this, I had to expose a few internal details of text elements; I think this is fine since Dart supports the @visibleForTesting annotation, and it makes the tests cleaner.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[spydon]

Lgtm!