Add strict types for file child

src/file/file-child.ts

@@ -0,0 +1,5 @@
+import { XmlComponent } from "@file/xml-components";
+
+export class FileChild extends XmlComponent {
+    public readonly fileChild = Symbol();
+}

src/file/file.ts

@@ -11,15 +11,13 @@ import { Footer, Header } from "./header";
 import { HeaderWrapper, IDocumentHeader } from "./header-wrapper";
 import { Media } from "./media";
 import { Numbering } from "./numbering";
-import { Paragraph } from "./paragraph";
 import { Comments } from "./paragraph/run/comment-run";
 import { Relationships } from "./relationships";
 import { Settings } from "./settings";
 import { Styles } from "./styles";
 import { ExternalStylesFactory } from "./styles/external-styles-factory";
 import { DefaultStylesFactory } from "./styles/factory";
-import { Table } from "./table";
-import { TableOfContents } from "./table-of-contents";
+import { FileChild } from "./file-child";
 
 export interface ISectionOptions {
     readonly headers?: {
@@ -33,7 +31,7 @@ export interface ISectionOptions {
         readonly even?: Footer;
     };
     readonly properties?: ISectionPropertiesOptions;
-    readonly children: readonly (Paragraph | Table | TableOfContents)[];
+    readonly children: readonly FileChild[];
 }
 
 export class File {

src/file/paragraph/paragraph.ts

@@ -1,7 +1,8 @@
 // http://officeopenxml.com/WPparagraph.php
 import { FootnoteReferenceRun } from "@file/footnotes";
-import { IContext, IXmlableObject, XmlComponent } from "@file/xml-components";
+import { IContext, IXmlableObject } from "@file/xml-components";
 import { uniqueId } from "@util/convenience-functions";
+import { FileChild } from "@file/file-child";
 
 import { TargetModeType } from "../relationships/relationship/relationship";
 import { DeletedTextRun, InsertedTextRun } from "../track-revision";
@@ -39,7 +40,7 @@ export interface IParagraphOptions extends IParagraphPropertiesOptions {
     readonly children?: readonly ParagraphChild[];
 }
 
-export class Paragraph extends XmlComponent {
+export class Paragraph extends FileChild {
     private readonly properties: ParagraphProperties;
 
     public constructor(options: string | IParagraphOptions) {

src/file/table-of-contents/table-of-contents.ts

@@ -3,13 +3,13 @@
 import { Paragraph } from "@file/paragraph";
 import { Run } from "@file/paragraph/run";
 import { Begin, End, Separate } from "@file/paragraph/run/field";
-import { XmlComponent } from "@file/xml-components";
+import { FileChild } from "@file/file-child";
 import { FieldInstruction } from "./field-instruction";
 import { StructuredDocumentTagContent } from "./sdt-content";
 import { StructuredDocumentTagProperties } from "./sdt-properties";
 import { ITableOfContentsOptions } from "./table-of-contents-properties";
 
-export class TableOfContents extends XmlComponent {
+export class TableOfContents extends FileChild {
     public constructor(alias: string = "Table of Contents", properties?: ITableOfContentsOptions) {
         super("w:sdt");
         this.root.push(new StructuredDocumentTagProperties(alias));

src/file/table/table.ts

@@ -1,5 +1,5 @@
 // http://officeopenxml.com/WPtableGrid.php
-import { XmlComponent } from "@file/xml-components";
+import { FileChild } from "@file/file-child";
 
 import { AlignmentType } from "../paragraph";
 import { TableGrid } from "./grid";
@@ -34,7 +34,7 @@ export interface ITableOptions {
     readonly visuallyRightToLeft?: boolean;
 }
 
-export class Table extends XmlComponent {
+export class Table extends FileChild {
     public constructor({
         rows,
         width,