Rename module

HighlightTags was bad because:
- we haven’t called placeholders ‘tags’ for a long time
- it also does resizing of the `<textarea>`, not just highlighting the
  placeholders

app/assets/javascripts/enhancedTextbox.js

@@ -0,0 +1,85 @@
+(function(Modules) {
+  "use strict";
+
+  if (
+    !('oninput' in document.createElement('input'))
+  ) return;
+
+  const tagPattern = /\(\(([^\)\((\?)]+)(\?\?)?([^\)\(]*)\)\)/g;
+
+  Modules.EnhancedTextbox = function() {
+
+    this.start = function(textarea) {
+
+      let visibleTextbox;
+
+      this.highlightPlaceholders = (
+        typeof textarea.data('highlightPlaceholders') === 'undefined' ||
+        !!textarea.data('highlightPlaceholders')
+      );
+
+      this.$textbox = $(textarea)
+        .wrap(`
+          <div class='textbox-highlight-wrapper' />
+        `)
+        .after(this.$background = $(`
+          <div class="textbox-highlight-background" aria-hidden="true" />
+        `))
+        .on("input", this.update);
+
+      visibleTextbox = this.$textbox.clone().appendTo("body").css({
+        position: 'absolute',
+        visibility: 'hidden',
+        display: 'block'
+      });
+      this.initialHeight = visibleTextbox.height();
+
+      this.$background.css({
+        'border-width': this.$textbox.css('border-width')
+      });
+
+      visibleTextbox.remove();
+
+      this.$textbox
+        .trigger("input");
+
+    };
+
+    this.resize = () => {
+
+      this.$background.width(this.$textbox.outerWidth());
+
+      this.$textbox.height(
+        Math.max(
+          this.initialHeight,
+          this.$background.outerHeight()
+        )
+      );
+
+      if ('stickAtBottomWhenScrolling' in GOVUK) {
+        GOVUK.stickAtBottomWhenScrolling.recalculate();
+      }
+
+    };
+
+    this.contentEscaped = () => $('<div/>').text(this.$textbox.val()).html();
+
+    this.contentReplaced = () => this.contentEscaped().replace(
+      tagPattern, (match, name, separator, value) => value && separator ?
+        `<span class='placeholder-conditional'>((${name}??</span>${value}))` :
+        `<span class='placeholder'>((${name}${value}))</span>`
+    );
+
+    this.update = () => {
+
+      this.$background.html(
+        this.highlightPlaceholders ? this.contentReplaced() : this.contentEscaped()
+      );
+
+      this.resize();
+
+    };
+
+  };
+
+})(window.GOVUK.Modules);