Add Themeable Color Icons to IIS Express Custom Tree View

[warrenbuckley]

This adds in coloured icons to the new IIS Express custom tree panel view that can be overriden by a user to fit into their styling/theme better if they wish to.

Overriding Icon Colors = Workbench or User Settings

"workbench.colorTheme": "Dracula Soft",
"workbench.colorCustomizations": {
    "iisexpress.startIcon": "#ff0000",
    "iisexpress.stopIcon": "#ff0000",
    "iisexpress.restartIcon": "#ff0000",
    "iisexpress.supporterIcon": "#ff0000"
}

Overriding Icon Colors = For a specific Color Theme

"workbench.colorCustomizations": {
    "[Visual Studio Light]": {
        "iisexpress.startIcon": "#ff0000",
        "iisexpress.stopIcon": "#ff0000",
        "iisexpress.restartIcon": "#ff0000",
        "iisexpress.supporterIcon": "#ff0000"
    }
}

⚠ This depends on VSCode merging this into the main product

This is using a proposed API from this issue I raised here for custom coloured CodIcons
microsoft/vscode#103120

API Proposal
microsoft/vscode#106491

[warrenbuckley]

Seems to be due for the October Release which is due on or around 4th Nov
microsoft/vscode#109419

[warrenbuckley]

Version 1.51 is now out, so need to update and create a new patch version for this