feat(designer-ui): Support raw code view in HTML editor

[ek68794998]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature: Adds a button to the HTML editor that allows switching to view and edit the raw HTML format. Resolves #3456.

• What is the current behavior? (You can also link to an open issue here)

Only rich editor is supported.

• What is the new behavior (if this is a feature change)?

Both rich editor and raw editor are supported.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No, not in terms of consumption interfaces, etc. This does introduce a number of changes to the HTML editor, though.

• Please Include Screenshots or Videos of the intended change: