[docs] Fix 3.52.0 a11y warning in svelte-self tutorial (#7988)

* fix svelte 3.52 a11y warning

* replace span with button and drop on:keyup

Author: janosh

site/content/tutorial/16-special-elements/01-svelte-self/app-a/Folder.svelte

@@ -10,7 +10,7 @@
 	}
 </script>
 
-<span class:expanded on:click={toggle}>{name}</span>
+<button class:expanded on:click={toggle}>{name}</button>
 
 {#if expanded}
 	<ul>
@@ -27,12 +27,14 @@
 {/if}
 
 <style>
-	span {
+	button {
 		padding: 0 0 0 1.5em;
 		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 {

site/content/tutorial/16-special-elements/01-svelte-self/app-b/Folder.svelte

@@ -10,7 +10,7 @@
 	}
 </script>
 
-<span class:expanded on:click={toggle}>{name}</span>
+<button class:expanded on:click={toggle}>{name}</button>
 
 {#if expanded}
 	<ul>
@@ -27,12 +27,14 @@
 {/if}
 
 <style>
-	span {
+	button {
 		padding: 0 0 0 1.5em;
 		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 {