feat: SAP icons in editor or hover

[uxkjaer]

Todo:

• [ ] Settings for showing icons in editor or only on hover
• [ ] Setting to disable icon hover or editor
• [ ] On hover functionality
• [ ] Code completion for icons
• [ ] Update readme with Icons functionality and prerequisite of SAP-icons fonts installed on machine
• [ ] Make tests pass
• [ ] Add integration tests

fixes #509

[lgtm-com[bot]]

This pull request introduces 5 alerts when merging f4effa82da3f3e881e617342e0b61d2272e2039c into 352164f58429c5535017db21bba628767bcb1708 - view on LGTM.com

new alerts:

• 5 for Unused variable, import, function or class

[uxkjaer]

Should the sap-icon font-family be added to the editor in the extension or should the user set it themselves in the settings? @petermuessig @bd82

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