Reduce amount of boilerplate to render enumerated preferences in the Chrome extension + improve linting

[Rob--W]

The number of preferences that are integers representing enumerations keeps growing. Boolean preferences are automatically dealt with, but integers/enumerations are not.
Currently, to support a preference in the Chrome extension, the following needs to be done:

1. extensions/chromium/preferences_schema.json should get a new entry. Preferences are only shown if a "title" is provided. Boolean prefs are automatically rendered, e.g.:
   

   pdf.js/extensions/chromium/preferences_schema.json

   Lines 45 to 49 in a89071b

   	"pdfBugEnabled": {
   	"title": "Enable debugging tools",
   	"description": "Whether to enable debugging tools.",
   	"type": "boolean",
   	"default": false

   
   The following format (enumerations) are becoming more common,:but unlike the boolean type, they need extra attention, explained below.
   

   pdf.js/extensions/chromium/preferences_schema.json

   Lines 29 to 38 in a89071b

   	"cursorToolOnLoad": {
   	"title": "Cursor tool on load",
   	"description": "The cursor tool that is enabled upon load.\n 0 = Text selection tool.\n 1 = Hand tool.",
   	"type": "integer",
   	"enum": [
   	0,
   	1
   	],
   	"default": 0
   	},

2. extensions/chromium/options/options.html For enumerated preferences, add a new template, e.g.
   

   pdf.js/extensions/chromium/options/options.html

   Lines 83 to 93 in a89071b

   	<template id="cursorToolOnLoad-template">
   	<div class="settings-row">
   	<label>
   	<span></span>
   	<select>
   	<option value="0">Text selection tool</option>
   	<option value="1">Hand tool</option>
   	</select>
   	</label>
   	</div>
   	</template>

3. extensions/chromium/options/options.js Add an if-check for the new preference name, and call a function that generates the event handler:
   

   pdf.js/extensions/chromium/options/options.js

   Lines 82 to 83 in a89071b

   	} else if (prefName === 'cursorToolOnLoad') {
   	renderPreference = renderCursorToolOnLoad(prefSchema.title);

   
   

   pdf.js/extensions/chromium/options/options.js

   Lines 203 to 218 in a89071b

   	function renderCursorToolOnLoad(shortDescription) {
   	var wrapper = importTemplate('cursorToolOnLoad-template');
   	var select = wrapper.querySelector('select');
   	select.onchange = function() {
   	storageArea.set({
   	cursorToolOnLoad: parseInt(this.value),
   	});
   	};
   	wrapper.querySelector('span').textContent = shortDescription;
   	document.getElementById('settings-boxes').appendChild(wrapper);
   	function renderPreference(value) {
   	select.value = value;
   	}
   	return renderPreference;
   	}

The problem here is that the enumeration type is common, yet it requires much boilerplate code (in options.js, a single helper function can be used). If we choose a consistent format, templates in options.html can be auto-generated too (at the expense of readability, because then the dropdown labels have to be written in preferences_schema.json).

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Secondly, to avoid breaking the settings page whenever a preference is changed/added, I should add linting to enforce that it is not possible to accidentally break the settings page (which has happened twice alredy, fixed in #8653 and #9490).