Accept relative patterns in NotebookDocumentFilter.is - #1931

NotebookDocumentFilter.is still only accepts a string pattern, although #1519 changed the pattern of all notebook filters to GlobPattern. The client converter already uses asGlobPattern for it, and TextDocumentFilter.is moved to GlobPattern.is in #1516. A notebook filter identified only by a relative pattern is rejected:

const rel = { baseUri: 'file:///ws', pattern: '**/*.ipynb' };
NotebookDocumentFilter.is({ pattern: '**/*.ipynb' }); // true
NotebookDocumentFilter.is({ pattern: rel });          // false, expected true

Because of this, NotebookCellTextDocumentFilter.is and DocumentSelector.is also return false. The client's asDocumentSelector then turns { notebook: { pattern: rel }, language: 'python' } into the plain text filter { language: 'python' }, or drops it when there is no language. This change uses GlobPattern.is for the notebook filter pattern, the same as TextDocumentFilter.is.

• Ran: npm run compile:protocol and the new protocol/src/node/test/documentFilter.test.ts. Before the change 4 of its 7 tests fail; after it all pass.
• Ran: npm --prefix protocol run test:node (12 passing) and npm --prefix protocol run lint (clean).
• Checked: string patterns still pass. {}, { pattern: 1 }, { pattern: {} } and an invalid baseUri are still rejected. A plain text filter with a relative pattern is still not treated as a notebook cell filter.