ZeroWidthSpace inserted when editing across lines

[lishid]

Similar to #691

When selecting across multiple lines and typing something to replace the selection, ZeroWidthSpace characters generated from replacements and widgets shows up.

• Open https://codemirror.net/6/ in Chrome.
• Run the following code block in dev console.
• Select from the first line to the second, just before the replacement (234\n1)
  .
• Type any character.
• Observe red dot "Control character zero width space" showing up.

const {EditorState, EditorView, basicSetup} = CM["@codemirror/basic-setup"];
const {WidgetType, Decoration, ViewPlugin, highlightSpecialChars} = CM["@codemirror/view"];
const {RangeSetBuilder} = CM["@codemirror/rangeset"];
const {StateField} = CM["@codemirror/state"];
let text = '1234\n1234\n1234';
let decos = Decoration.set([Decoration.replace({}).range(6,8)]);
let plugin = ViewPlugin.define(view => {}, {decorations: v => decos});
let state = EditorState.create({
	doc: text,
	extensions: [basicSetup, highlightSpecialChars(), plugin]
});
view.setState(state);

[marijnh]

Heh, damn, I knew we'd have bugs like this. Attached patch makes the zero-width space stripping more aggressive.