Improve render performance (#511)

Massive improvement of render performance by:
- replacing the codimd-line-marker with an in-memory map
- an observation of the changed markdown code to identify changed lines
- a unique react-key calculation

src/components/markdown-renderer/replace-components/highlighted-fence/highlighted-code/highlighted-code.tsx

@@ -20,7 +20,7 @@ export const escapeHtml = (unsafe: string): string => {
 }
 
 const checkIfLanguageIsSupported = (language: string): boolean => {
-  return hljs.listLanguages().indexOf(language) > -1
+  return hljs.listLanguages().includes(language)
 }
 
 const correctLanguage = (language: string | undefined): string | undefined => {

src/components/markdown-renderer/replace-components/highlighted-fence/highlighted-fence-replacer.tsx

@@ -3,10 +3,10 @@ import React from 'react'
 import { ComponentReplacer } from '../ComponentReplacer'
 import { HighlightedCode } from './highlighted-code/highlighted-code'
 
-export class HighlightedCodeReplacer implements ComponentReplacer {
+export class HighlightedCodeReplacer extends ComponentReplacer {
   private lastLineNumber = 0;
 
-  getReplacement (codeNode: DomElement, index: number): React.ReactElement | undefined {
+  public getReplacement (codeNode: DomElement, index: number): React.ReactElement | undefined {
     if (codeNode.name !== 'code' || !codeNode.attribs || !codeNode.attribs['data-highlight-language'] || !codeNode.children || !codeNode.children[0]) {
       return
     }