add keepLines and keepNext support for paragraph formatting

This commit adds it to the three places where paragraph formatting can
be applied right now (directly on the paragraph, paragraph styles, and
bullets/numbering styles). I'm separately adding an entry to the wiki
regarding what these methods do (widow/orphan control)

ts/tests/docx/paragraph/paragraphTests.ts

@@ -269,4 +269,24 @@ describe("Paragraph", () => {
             });
         });
     });
+
+    describe("#keepLines", () => {
+        it("should set the paragraph keepLines sub-component", () => {
+            paragraph.keepLines();
+            const tree = new Formatter().format(paragraph);
+            expect(tree).to.deep.equal({
+                "w:p": [{"w:pPr": [{"w:keepLines": []}]}],
+            });
+        });
+    });
+
+    describe("#keepNext", () => {
+        it("should set the paragraph keepNext sub-component", () => {
+            paragraph.keepNext();
+            const tree = new Formatter().format(paragraph);
+            expect(tree).to.deep.equal({
+                "w:p": [{"w:pPr": [{"w:keepNext": []}]}],
+            });
+        });
+    });
 });