feat: import markdown-it-plugins from https://github.com/hedgedoc/markdown-it-plugins

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

markdown-it-plugins/README.md

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+<!--
+SPDX-FileCopyrightText: 2020 The HedgeDoc developers (see AUTHORS file)
+
+SPDX-License-Identifier: CC0-1.0
+-->
+
+# Markdown-It plugins
+
+This repository contains plugins for [Markdown-It](https://github.com/markdown-it/markdown-it) that are used by HedgeDoc.
+All these plugins are (re)created in typescript.
+
+## License
+Everything is licensed under MIT
+
+## Usage
+Install the lib using `yarn install @hedgedoc/markdown-it-plugins` or `npm i @hedgedoc/markdown-it-plugins`
+
+## Development
+If you want to contribute to this lib then:
+- Clone this repository
+- Install the dependencies using `yarn install`. Don't use `npm`!
+- Make your changes
+- Make sure that your changes are covered by tests. Use `yarn test` to run all tests
+- Make sure that your code follows the code style. Use `yarn lint` to check the style
+- Commit your changes (please use [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/)) and create a pull request
+
+## markdown-it-image-size
+A markdown-it plugin for size-specified image markups. This plugin overloads the original image renderer of markdown-it.
+> This is a typescript port of https://github.com/tatsy/markdown-it-imsize without the local file system support.
+
+### Usage
+
+#### Enable plugin
+
+```ts
+import MarkdownIt from 'markdown-it'
+import { imageSize } from '@hedgedoc/markdown-it-plugins'
+
+const md = new MarkdownIt({
+    html: true,
+    linkify: true,
+    typography: true
+}).use(imageSize);
+```
+
+#### Example
+
+```md
+![test](image.png =100x200)
+```
+
+is interpreted as
+
+```html
+<p><img src="image.png" alt="test" width="100" height="200"></p>
+```
+
+## markdown-it-better-task-lists
+
+A markdown-it plugin that renders GitHub-style task-lists. It builds [task/todo lists](https://github.com/blog/1825-task-lists-in-all-markdown-documents) out of Markdown lists with items starting with `[ ]` or `[x]`.
+
+This is a typescript port of [markdown-it-task-lists](https://github.com/revin/markdown-it-task-lists).
+
+### Why is this useful?
+
+When you have markdown documentation with checklists, rendering HTML checkboxes
+out of the list items looks nicer than the raw square brackets.
+
+### Usage
+
+Use it the same as a normal markdown-it plugin:
+
+```ts
+import MarkdownIt from 'markdown-it'
+import { taskLists } from '@hedgedoc/markdown-it-plugins'
+
+const parser = new MarkdownIt().use(taskLists)
+
+const result = parser.render(`
+- [ ] Open task
+- [x] Done task
+- Not a task
+`) // markdown string containing task list items
+```
+
+The rendered checkboxes are disabled; to change this, set `enabled` property of the
+plugin options to `true`:
+
+```ts
+const parser = new MarkdownIt().use(taskLists, { enabled: true })
+```
+
+If you need to know which line in the markdown document the generated checkbox comes
+set the `lineNumber` property of the plugin options to `true` for the
+`<input>` tag to be created with a data-line attribute containing the line number:
+
+```ts
+const parser = new MarkdownIt().use(taskLists, { lineNumber: true })
+```
+
+If you'd like to wrap the rendered list items in a `<label>` element for UX
+purposes, set the `label` property of the plugin options to `true`:
+
+```ts
+const parser = new MarkdownIt().use(taskLists, { label: true })
+```
+
+To add the label after the checkbox set the `labelAfter` property of the plugin
+options to `true`:
+
+```ts
+const parser = new MarkdownIt().use(taskLists, { label: true, labelAfter: true })
+```
+
+**Note:** This option does require the `label` option to be truthy.
+
+The options can be combined, of course.
+
+## markdown-it-toc
+
+A markdown-it plugin that renders a table of contents.
+It uses the found headlines as content.
+
+This is a typescript port of [markdown-it-toc-done-right](https://github.com/nagaozen/markdown-it-toc-done-right).
+
+### Usage
+
+Use it the same as a normal markdown-it plugin:
+
+```ts
+import MarkdownIt from 'markdown-it'
+import { toc } from '@hedgedoc/markdown-it-plugins'
+
+const parser = new MarkdownIt().use(toc)
+
+const result = parser.render(`
+[toc]
+
+# head 1
+
+# head 2
+`) // markdown string containing task list items
+```