Provide file/folder completion for certain xml attributes

[fbricon]

When an attribute is named:

• src
• path
• directory
• file
  (list could be defined in some preferences), then lsp4xml should propose file completion.
  Also, that could be triggered by any attribute value starting with the file:// scheme.

eg (html support in vscode):

[bmix]

What about adding support for the xml:base attribute, valid for the current XML context, as root for the completion?

xml:base allows for links to be relative, which then get resolved by an iri-handler, by concatenating the path from the currently valid xml:base attribute with the relative reference.

Example (let the file in edition be /projects/clientA/documents/example.xml):

<document xml:base="http://example.com/documents/">
  <title>Some Title</title>
  <icon href="icon.png"/> <!-- means: 'http://example.com/documents/icon.png' -->
  <text>Lorem Ipsum and so on...</text>
  <figures xml:base="http://example.com/documents/figures/">
    <fig href="-> This triggers listing all images under '/documents/figures/'
  </figures>
</document>

Of course, this would make only sense for environments, that implement some logic to map any non file:// IRI to the local filesystem:

settings.remote.path.url = "http://example.com"
settings.local.path.url = "/projects/clientA/"

Such settings would typically be part of the project's preferences.

Resolution of the currently valid such attribute would, I believe, require the same code, as may be existing for namespace resolution, xml:lang and xml:space, because the principles are the same.