Customizing list icons: give a way to apply to current item only

[KevinEyo1]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
Fixes #2405
Modified CustomListIconProcessor.ts to account for new once variable.
Updated lists UG file to include new feature.
Update testLists to test new feature.

Anything you'd like to highlight/discuss:
Implemented such that it will not overwrite previous icon saved.

1. Customization + one-off + none
* Item A { icon="fas-file-code" }
* Item B { icon="glyphicon-education" once=true }
  * Sub-item B1
  * Sub-item B2
  * Sub-item B3 
    * Sub-sub-item B3.1 { icon="./images/deer.jpg" i-width="50px" }
    * Sub-sub-item B3.2
  * Sub-item B4 
* Item C

Currently once icon being in the first item, AND the next item having no icon, results in a visual issue.
To note that the final item with a new icon is working as intended.

1. One-off + none + customization
* Item A { icon="glyphicon-education" once=true }
* Item B 
  * Sub-item B1 { icon="fas-file-code" }
  * Sub-item B2
  * Sub-item B3 
    * Sub-sub-item B3.1 { icon="./images/deer.jpg" i-width="50px" }
    * Sub-sub-item B3.2
  * Sub-item B4 
* Item C { icon="fas-file-code" }

Testing instructions:
Serve the folder at markbind\packages\cli\test\functional\test_site and navigate to testList.html

Proposed commit message: (wrap lines at 72 characters)
List icons: icons for specific item only

Unordered list icons are used in subsequent items to avoid
duplication.

MarkBind lacks a way to indicate icons for a specific item
only, limiting the flexibility of icons.

Creating a variable to indicate once icons allow for user
to have greater flexibility while preserving the current nature
of list icons.

Let's add a once variable that users can set when adding
icons to items in the list, such that future items won't inherit
the icon, and it will also not overwrite previous icons saved.

This approach directly allows users to pick specific icons for
specific items, while keeping the inheritance nature of other icons
intact.

---

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

---

Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[yucheng11122017]

Small nits

[yucheng11122017]

Suggested change

	// If an item is once, its icons and/or attributes will only be used for that item.
	// If `once` is true, its icons and/or attributes will only be used for that item.

[yucheng11122017]

Suggested change

	// for items after first item, if first item is once, no previous icon
	// for subsequent items, if first item is once, there is no previous icon

[yucheng11122017]

Suggested change

	// Save if current has icon or text, and it is not once
	// Save if current item has icon or text, and it is not once

[yucheng11122017]

Suggested change

	// update only if current has icon/text or previous has saved icon/text
	// update only if current item has icon/text or previous item has saved icon/text

[yucheng11122017]

Suggested change

	// so future items that are not once will need to be saved
	// so future attributes that are not once will need to be saved

[yucheng11122017]

LGTM! Thanks for all the work @KevinEyo1