docs: add svelte:element/document chapters (sveltejs#270)

from old tutorial - also updates some existing content

Author: dummdidumm

content/tutorial/03-advanced-svelte/09-special-elements/01-svelte-self/app-a/src/lib/Folder.svelte

@@ -10,9 +10,7 @@
 	}
 </script>
 
-<span class:expanded on:click={toggle}
-	>{name}</span
->
+<button class:expanded on:click={toggle}>{name}</button>
 
 {#if expanded}
 	<ul>
@@ -29,13 +27,14 @@
 {/if}
 
 <style>
-	span {
+	button {
 		padding: 0 0 0 1.5em;
-		background: url(/tutorial/icons/folder.svg) 0
-			0.1em no-repeat;
+		background: url(/tutorial/icons/folder.svg) 0 0.1em no-repeat;
 		background-size: 1em 1em;
 		font-weight: bold;
 		cursor: pointer;
+		border: none;
+		margin: 0;
 	}
 
 	.expanded {

content/tutorial/03-advanced-svelte/09-special-elements/01-svelte-self/app-b/src/lib/File.svelte

@@ -10,9 +10,7 @@
 	}
 </script>
 
-<span class:expanded on:click={toggle}
-	>{name}</span
->
+<button class:expanded on:click={toggle}>{name}</button>
 
 {#if expanded}
 	<ul>
@@ -29,13 +27,14 @@
 {/if}
 
 <style>
-	span {
+	button {
 		padding: 0 0 0 1.5em;
-		background: url(/tutorial/icons/folder.svg) 0
-			0.1em no-repeat;
+		background: url(/tutorial/icons/folder.svg) 0 0.1em no-repeat;
 		background-size: 1em 1em;
 		font-weight: bold;
 		cursor: pointer;
+		border: none;
+		margin: 0;
 	}
 
 	.expanded {

content/tutorial/03-advanced-svelte/09-special-elements/01-svelte-self/app-b/src/lib/Folder.svelte

@@ -0,0 +1,23 @@
+---
+title: <svelte:element>
+---
+
+Sometimes we don't know in advance what kind of DOM element to render. `<svelte:element>` comes in handy here. Instead of a sequence of `if` blocks...
+
+```html
+{#if selected === 'h1'}
+	<h1>I'm a h1 tag</h1>
+{:else if selected === 'h3'}
+	<h3>I'm a h3 tag</h3>
+{:else if selected === 'p'}
+	<p>I'm a p tag</p>
+{/if}
+```
+
+...we can have a single dynamic component:
+
+```html
+<svelte:element this={selected}>I'm a {selected} tag</svelte:element>
+```
+
+The `this` value can be any string, or a falsy value — if it's falsy, no element is rendered.

content/tutorial/03-advanced-svelte/09-special-elements/03-svelte-element/README.md

@@ -0,0 +1,18 @@
+<script>
+	const options = ['h1', 'h3', 'p'];
+	let selected = options[0];
+</script>
+
+<select bind:value={selected}>
+	{#each options as option}
+		<option value={option}>{option}</option>
+	{/each}
+</select>
+
+{#if selected === 'h1'}
+	<h1>I'm a h1 tag</h1>
+{:else if selected === 'h3'}
+	<h3>I'm a h3 tag</h3>
+{:else if selected === 'p'}
+	<p>I'm a p tag</p>
+{/if}

content/tutorial/03-advanced-svelte/09-special-elements/03-svelte-element/app-a/src/lib/App.svelte

@@ -0,0 +1,12 @@
+<script>
+	const options = ['h1', 'h3', 'p'];
+	let selected = options[0];
+</script>
+
+<select bind:value={selected}>
+	{#each options as option}
+		<option value={option}>{option}</option>
+	{/each}
+</select>
+
+<svelte:element this={selected}>I'm a {selected} tag</svelte:element>