feat(block): added FillerTemplate to block

[dusktreader]

Prerequisites

• I have read and understood the [contributing guide][CONTRIBUTING.md].
• The commit message follows the [conventional commits][cc] guidelines.
• Tests for the changes have been added (for bug fixes / features).
• Docs have been added/updated (for bug fixes / features).

Description

This change introduces a new block configuration called "FillerTemplate". This allows the user to specify a template for filler so that different behavior can be configured when the block is handling an overflow.

An example configuration might be:

- type: prompt
  alignment: right
  filler_template: "{{ if .Overflow }} {{ else }}<#3d59a1,transparent>━</>{{ end }}"
  overflow: break

This would draw filler text ("-") when there is no overflow and empty space when there is an overflow.

[dusktreader]

Example renderings:

Given config:

- type: prompt
  alignment: right
  filler_template: "{{ if .Overflow }} {{ else }}<#3d59a1,transparent>━</>{{ end }}"
  overflow: break

No Overflow:

With Overflow:

[JanDeDobbeleer]

@dusktreader you could also enable filler to be template-able, it doesn't need an additional property. I'll refactor this a bit so you can see how that would work.

[JanDeDobbeleer]

@all-contributors please add @dusktreader for code,doc

This is pure magic 🪄! @holopin-bot @dusktreader wizard

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[allcontributors]

@JanDeDobbeleer

I've put up a pull request to add @dusktreader! 🎉

[dusktreader]

Thanks! Your refactor is definitely a better approach. I'm excited for the next release!