autofix some eslint unicorn/prefer-string-slice

I removed a couple of the autofixes because I don't like how it added Math.max(), which is complicated to read. I kept the helpful ones.

.eslintrc.json

@@ -27,7 +27,6 @@
 		"space-before-function-paren": "off",
 		"space-in-parens": "off",
 		"template-curly-spacing": "off",
-		"unicorn/prefer-string-slice": "off",
 
 		"no-nested-ternary": "error",
 		"no-restricted-syntax": [
@@ -62,6 +61,7 @@
 		"no-use-before-define": "warn",
 		"no-useless-concat": "warn",
 		"no-var": "warn",
-		"prefer-const": "warn"
+		"prefer-const": "warn",
+		"unicorn/prefer-string-slice": "warn"
 	}
 }

modules/twinkleconfig.js

@@ -1496,7 +1496,7 @@ Twinkle.config.listDialog.save = function twinkleconfigListDialogSave($button, $
 // reset/restore defaults
 
 Twinkle.config.resetPrefLink = function twinkleconfigResetPrefLink(e) {
-	const wantedpref = e.target.id.substring(21); // "twinkle-config-reset-" prefix is stripped
+	const wantedpref = e.target.id.slice(21); // "twinkle-config-reset-" prefix is stripped
 
 	// search tactics
 	$(Twinkle.config.sections).each((sectionkey, section) => {

modules/twinklewarn.js

@@ -1918,7 +1918,7 @@ Twinkle.warn.callbacks = {
 		const customProcess = function(template) {
 			template = template.split('|')[0];
 			let prefix;
-			switch (template.substr(-1)) {
+			switch (template.slice(-1)) {
 				case '1':
 					prefix = 'General note';
 					break;
@@ -1932,7 +1932,7 @@ Twinkle.warn.callbacks = {
 					prefix = 'Final warning';
 					break;
 				case 'm':
-					if (template.substr(-3) === '4im') {
+					if (template.slice(-3) === '4im') {
 						prefix = 'Only warning';
 						break;
 					}
@@ -1950,9 +1950,9 @@ Twinkle.warn.callbacks = {
 		} else {
 			// Normalize kitchensink to the 1-4im style
 			if (params.main_group === 'kitchensink' && !/^D+$/.test(params.sub_group)) {
-				let sub = params.sub_group.substr(-1);
+				let sub = params.sub_group.slice(-1);
 				if (sub === 'm') {
-					sub = params.sub_group.substr(-3);
+					sub = params.sub_group.slice(-3);
 				}
 				// Don't overwrite uw-3rr, technically unnecessary
 				if (/\d/.test(sub)) {

morebits.js

@@ -1421,15 +1421,15 @@ Morebits.string = {
 	 */
 	toUpperCaseFirstChar: function(str) {
 		str = str.toString();
-		return str.substr(0, 1).toUpperCase() + str.substr(1);
+		return str.slice(0, 1).toUpperCase() + str.slice(1);
 	},
 	/**
 	 * @param {string} str
 	 * @return {string}
 	 */
 	toLowerCaseFirstChar: function(str) {
 		str = str.toString();
-		return str.substr(0, 1).toLowerCase() + str.substr(1);
+		return str.slice(0, 1).toLowerCase() + str.slice(1);
 	},
 
 	/**
@@ -4830,7 +4830,7 @@ Morebits.wikitext.parseTemplate = function(text, start) {
 	function findParam(final) {
 		// Nothing found yet, this must be the template name
 		if (count === -1) {
-			result.name = current.substring(2).trim();
+			result.name = current.slice(2).trim();
 			++count;
 		} else {
 			// In a parameter