feat: Structured text and text styles

[st-pasha]

Description

This PR introduces the notions of structured text, and text styles, to support rendering of rich text bodies.

Specifically, we recognize that sometimes in games one needs to render pieces of text that are larger than a single word or even a single paragraph. These pieces may include: books, quest descriptions, mission objectives, tutorials, in-game help system, dialogues, etc. Rendering such a piece of text is non-trivial, however. In order to tackle this problem, I break into the following parts:

1. Text structure, represented as a tree of Nodes. The nodes describe the logical structure of the text, for example the document may contain a header, and then several paragraphs, and a list, where the list contains some list items, some of which having possibly several paragraphs, etc. This structure is similar to how in HTML the text is marked up with HTML tags.

2. Text styles are struct-like classes that contain properties describing how the text is to be styled: font size, font renderer, borders, backgrounds, margins, padding, etc. This representation is also tree-like, so that for example text inside paragraphs can have different style than text within headers, and paragraphs within lists can have different margins. A text style is similar to a stylesheet in HTML.

3. Text elements are the result of applying the document style to a document node: they are the "prepared" and laid out pieces, ready to be rendered. Elements are a bit like mini-components, or perhaps text "particles" in a particle system.

Screenshot of the example:

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• 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.

Breaking Change

• [-] Yes, this is a breaking change.
• No, this is not a breaking change.

Related Issues

WIP for #1627

[erickzanardo]

@st-pasha can you attach a screen shoot of the example of this PR?

[spydon]

Lgtm, just one minor comment