Add a file upload plugin

Solving #8
Of the 3 proposed solutions in #8, the first is limited by size, and the  third one was complicated to implement inside a React app with shacl-form's shadow DOM.
So, the plugin option was the best answer to this issue.
The goal of this plugin is to pass a lambda as a parameter that handles  the file upload.
E.g., in a React app:
```ts
const [myFile, setMyFile] = useState<File>();
form.registerPlugin(new FileUploadPlugin({datatype: '...'}, (event) => setMyFile(event.target.files?.[0])));
```
Potential issue: if the form has multiple file upload inputs, and the user submits two different files with the same name (like `joe/profile.png` and `jane/profile.png`), the result will be, for both images, the fake path of the browser: `C:\fakepath\profile.png`. This will make it hard to distinguish between them after form submission. Something needs to be implemented here, to be investigated..

src/plugins/file-upload.ts

@@ -0,0 +1,30 @@
+import { Plugin, PluginOptions } from '../plugin'
+import { Term } from '@rdfjs/types'
+
+import { ShaclPropertyTemplate } from '../property-template'
+import {  InputListEntry } from '../theme'
+
+export class FileUploadPlugin extends Plugin {   
+  	onChange: any
+    constructor(options: PluginOptions, onChange: (event: any) => void, fileType?: string) {
+        super(options)
+      	this.onChange = onChange
+    }
+
+    createEditor(template: ShaclPropertyTemplate, value?: Term ): HTMLElement {
+        const required = template.minCount !== undefined && template.minCount > 0
+        let editor
+        editor = document.createElement('input')
+        editor.type = 'file'
+        if (fileType)
+          editor.setAttribute('accept', filetype) 
+      	editor.addEventListener('change', this.onChange);
+
+        return template.config.theme.createDefaultTemplate(
+        	template.label,
+        	value || null,
+        	required,
+        	editor,
+        	template)
+    }
+}

webpack.config.js

@@ -9,6 +9,7 @@ module.exports = [
             'plugins/mapbox': './src/plugins/mapbox.ts',
             'plugins/leaflet': './src/plugins/leaflet.ts',
             'plugins/fixed-list': './src/plugins/fixed-list.ts',
+            'plugins/file-upload': './src/plugins/file-upload.ts',
         },
         experiments: { outputModule: true },
         output: {
@@ -30,4 +31,4 @@ module.exports = [
         },
         // devtool: "source-map",
     },
-];
+];