Enable start server from absolute/relative filename

[andreamah]

Relevant to #384

livePreview.start.preview.atFile used to take a string argument for the path, but takes a URI input now because of some extension cleanup. This means that a tasks.json that looks like:

{
    "version": "2.0.0",
    "tasks": [        {
            "label": "test",
            "command": "${input:preview-test}",
            "problemMatcher": []
        }],
    "inputs": [
        {
            "id": "preview-test",
            "type": "command",
            "command": "livePreview.start.preview.atFile",
            "args": "any-file.html"
        }
    ]
}

won't work.

This feature request proposes that there is a new command that takes in a string to preview such that extensions and users can use with schemes that can only take simple inputs.

[andreamah]

Verif steps:

Create a tasks.json like this:

{
	"version": "2.0.0",
	"tasks": [
		{
			"label": "Preview Page",
			"command": "${input:preview-args}",
			"problemMatcher": []
		}
	],
	"inputs": [
		{
			"id": "preview-args",
			"type": "command",
			"command": "livePreview.start.preview.atFileString",
			"args": "{path_to_preview}"
		}
	]
}

Where the path to preview can be an absolute or relative path. Then, run the Preview Page to view the page that you linked.