add xml:space="preserve" to text runs

In MS Word 2015 (and possibly others), leading and trailing spaces are
ignored in text runs. This means that calling TextRun with
leading/trailing space would result in a document that didn't include
those spaces. The fix here (per http://officeopenxml.com/WPtext.php)
is to include an extra attribute on the "w:t" element that forces word
to recognize those spaces.

ts/tests/docx/paragraph/paragraphTests.ts

@@ -39,7 +39,7 @@ describe("Paragraph", () => {
             expect(tree).to.be.an("array").which.includes({
                 "w:r": [
                     {"w:rPr": []},
-                    {"w:t": ["this is a test run"]},
+                    {"w:t": [{_attr: {"xml:space": "preserve"}}, "this is a test run"]},
                 ],
             });
         });