File association should support relative path for systemId

[fbricon]

right now, trying to validate the examples from IBM's XML language server by associating files like:

"xml.fileAssociations": [{
    "systemId": "file_association/invoice.xsd",
    "pattern": "**/file_association/invoice_*.xml"
},

has no effect. I tried using an absolute path for systemId but didn't work either (tried on a Mac)

[fbricon]

so absolute path actually works but I had to restart the server to get it working.

Also, after using a relative path and restarting the server, I get a bunch of errors like:

schema_reference.4: Failed to read schema document 'file_association/invoice.xsd', because 1) could not find the document; 2) the document could not be read; 3) the root element of the document is not xsd:schema.

[angelozerr]

@fbricon please tell me if it works for you.

[fbricon]

Works for **/file_association/invoice_*.xml. Not for */file_association/invoice_*.xml (the syntax in IBM's example). Not sure if it's a big deal

[angelozerr]

The IBM Language Server uses regexp to do that. In my case I'm using glob pathmatcher. I'm not sure if we should support IBM Language Server syntax too.

[fbricon]

It's probably fine as-is