Schema modeline should also resolve schema path from project root or multi-project root

Describe the bug

In a multi-root workspace, there's a schema

def/bla_schema.json

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "bla": {
      "type": "array",
      "items": {
        "type": "string",
        "enum": ["foo", "bar", "fla"]
      }
    }
  },
  "required": ["bla"],
  "additionalProperties": false
}

In another project there's a file

use/bla.json

# yaml-language-server: $schema=def/bla_schema.json

bla:
  - foo
  - bar
  - fla
  - foo

Even though README says that paths are resolved wrt multi-project roots, validating just errors out.

Now trying a relative path over workspace roots:

# yaml-language-server: $schema=../def/bla_schema.json

Expected result

Path is resolved to <prefix>/use/../def/bla_schema.json and autocompletion is working.

Actual result

Path is resolved to <prefix>/use/def/bla_schema.json, thus yielding an invalid location. Obviously, autocompletion does not work.

Unable to load schema from '<prefix>/use/def/bla_schema.json': No content.YAML(768)

Proposed solution

Either

1. support paths relative to workspace roots, or
2. retain .. parts in modeline.

Environment

• Windows
• Mac
• Linux (WSL2)
• other (please specify)

Start with the README's documented path-resolution behavior and trace the schema modeline handling used for multi-root workspaces. Reproduce the provided def/bla_schema.json and use/bla.json layout, then verify that validation and autocompletion resolve both root-relative and .. paths to the intended schema.