Add getSpecialCharacters for escaping of extension characters

...monmark/ext/gfm/strikethrough/StrikethroughExtension.java

.../gfm/strikethrough/StrikethroughMarkdownRendererTest.java

...n/java/org/commonmark/ext/gfm/tables/TablesExtension.java

.../main/java/org/commonmark/internal/util/AsciiMatcher.java

...ommonmark/renderer/markdown/CoreMarkdownNodeRenderer.java

...onmark/renderer/markdown/MarkdownNodeRendererContext.java

...onmark/renderer/markdown/MarkdownNodeRendererFactory.java

...va/org/commonmark/renderer/markdown/MarkdownRenderer.java