feat(Prism addon): Add prisom addon for syntax highlighting

src/addons/prism/README.md

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+# Prism addon
+
+Syntax highlighting for `CodeFragment` and `CodeBlock` nodes using [Prism](https://prismjs.com/). You can also use this addon to style executable node types like `CodeExpression` and `CodeChunk` if you do not use the `stencila-components` addon.
+
+## Notes
+
+- Currently this addon renders syntax highlighting in the browser. In the future, we may take a similar approach as for MathJax and pre-render HTML in Encoda. See "Usage with Node" on https://prismjs.com/.
+
+- Currently, syntax highlighting is only enabled for a limited number of languages. There is a https://prismjs.com/plugins/autoloader/ plugin but that would not be usable with are Parcel bundling approach. That is one reason in favor of doing pre-rending in Encoda - a reader would not have to load the Javascript to every language.
+
+- There are many [plugins](https://prismjs.com/plugins) that may be appropriate to either add here, or as other addons e.g. `prism-line-numbers`.

src/addons/prism/index.ts

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+import 'prismjs'
+import 'prismjs/components/prism-python'
+import 'prismjs/components/prism-r'

src/addons/prism/styles.css

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+@import 'prismjs/themes/prism.css';