Enforce Go style rules across codebase

Apply five style rules: convert iota string enums to typed
string constants, replace errors.As with errors.AsType,
merge three-group imports into two groups, fix multiline
parameter/argument formatting, and replace fmt.Sprintf URL
construction with net/url.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/prosemirror/markdown_converter.go

@@ -635,11 +635,14 @@ func (c *converter) convertTableCells(row ast.Node, cellType NodeType) ([]Node,
 			return nil, fmt.Errorf("cannot marshal table cell attrs: %w", err)
 		}
 
-		cells = append(cells, Node{
-			Type:    cellType,
-			Attrs:   attrs,
-			Content: content,
-		})
+		cells = append(
+			cells,
+			Node{
+				Type:    cellType,
+				Attrs:   attrs,
+				Content: content,
+			},
+		)
 	}
 
 	return cells, nil