Add support for null for root types

Closes GH-6.

Reviewed-by: Titus Wormer <[email protected]>

Author: ChristianMurphy

types/index.d.ts

@@ -1,6 +1,6 @@
 // TypeScript Version: 3.7
 
-import {Element, Node} from 'xast'
+import {Element, Node, Root} from 'xast'
 
 type Children = string | Node | number | Children[]
 
@@ -19,6 +19,14 @@ type Attributes = Record<string, Primitive>
  */
 declare function xastscript(name: string, ...children: Children[]): Element
 
+/**
+ * Create XML trees in xast.
+ *
+ * @param name Qualified name. Case sensitive and can contain a namespace prefix (such as rdf:RDF).
+ * @param children (Lists of) child nodes. When strings are encountered, they are mapped to Text nodes.
+ */
+declare function xastscript(name: null, ...children: Children[]): Root
+
 /**
  * Create XML trees in xast.
  *

types/test.ts

@@ -2,12 +2,17 @@ import x = require('xastscript')
 
 x('urlset') // $ExpectType Element
 x('urlset', 'string') // $ExpectType Element
+x('urlset', 1) // $ExpectType Element
 x('urlset', ['string', 'string']) // $ExpectType Element
 x('urlset', x('loc'), 'string') // $ExpectType Element
 x('urlset', x('loc')) // $ExpectType Element
 x('urlset', x('loc'), x('loc')) // $ExpectType Element
 x('urlset', [x('loc'), x('loc')]) // $ExpectType Element
 x('urlset', []) // $ExpectType Element
+x(null) // $ExpectType Root
+x(null, 'string') // $ExpectType Root
+x(null, 1) // $ExpectType Root
+x(null, []) // $ExpectType Root
 
 const xmlns = 'http://www.sitemaps.org/schemas/sitemap/0.9'
 
@@ -30,3 +35,4 @@ x('urlset', {xmlNumberAttribute}, []) // $ExpectType Element
 x() // $ExpectError
 x(false) // $ExpectError
 x('urlset', x('loc'), {xmlns}) // $ExpectError
+x(null, {xmlns}) // $ExpectError