sveltekit-prototype: Upgrade to Svelte 4.0 (#54603)

Announcement: https://svelte.dev/blog/svelte-4


## Test plan

```
cd client/web-sveltekit
pnpm dev
```

Dev build loads successfully in browser

client/web-sveltekit/package.json

@@ -17,22 +17,22 @@
 	},
 	"devDependencies": {
 		"@playwright/test": "1.25.0",
-		"@sveltejs/adapter-auto": "^2.0.0",
-		"@sveltejs/adapter-static": "^2.0.0",
-		"@sveltejs/kit": "^1.5.6",
+		"@sveltejs/adapter-auto": "^2.1.0",
+		"@sveltejs/adapter-static": "^2.0.2",
+		"@sveltejs/kit": "^1.20.4",
 		"@types/cookie": "^0.5.1",
 		"@types/prismjs": "^1.26.0",
 		"eslint-plugin-svelte3": "^4.0.0",
-		"prettier-plugin-svelte": "^2.9.0",
-		"svelte": "^3.55.1",
-		"svelte-check": "^3.0.4",
+		"prettier-plugin-svelte": "^2.10.1",
+		"svelte": "^4.0.0",
+		"svelte-check": "^3.4.3",
 		"tslib": "2.1.0",
-		"vite": "^4.1.4"
+		"vite": "^4.3.9"
 	},
 	"type": "module",
 	"dependencies": {
-		"@popperjs/core": "^2.11.6",
-		"@remix-run/router": "^1.3.2",
+		"@popperjs/core": "^2.11.8",
+		"@remix-run/router": "^1.6.3",
 		"@sourcegraph/branded": "workspace:*",
 		"@sourcegraph/common": "workspace:*",
 		"@sourcegraph/http-client": "workspace:*",
@@ -42,4 +42,4 @@
 		"lodash-es": "^4.17.21",
 		"prismjs": "^1.29.0"
 	}
-}
+}

client/web-sveltekit/src/lib/Popover.svelte

@@ -37,7 +37,7 @@
 <slot {toggle} {registerTrigger} />
 {#if isOpen}
     <div class="content" bind:this={content} use:onClickOutside on:click-outside={clickOutside}>
-        <slot name="content" />
+        <slot name="content" {toggle} />
     </div>
 {/if}
 

client/web-sveltekit/src/lib/search/SearchBox.svelte

@@ -95,7 +95,7 @@
                 <Icon svgPath={mdiLightningBolt} inline />
             </button>
         </Tooltip>
-        <div slot="content" class="popover-content">
+        <div slot="content" class="popover-content" let:toggle>
             {@const delayedClose = () => setTimeout(() => toggle(false), 100)}
             <div class="d-flex align-items-center px-3 py-2">
                 <h4 class="m-0 mr-auto">SmartSearch</h4>

client/web-sveltekit/src/routes/Header.svelte

@@ -69,12 +69,6 @@
         flex: 1;
     }
 
-    svg {
-        width: 1rem;
-        height: 1rem;
-        margin-right: 0.5rem;
-    }
-
     ul {
         position: relative;
         padding: 0;