UX: Multichain: Add separator under 'View on Explorer' (#21223)

## **Description**
This PR adds a separator to the Multichain global menu under View on
Explorer

## **Manual testing steps**

1.  Open the Global Menu
2. See the separator between View on Explorer and its next menu item

## **Screenshots/Recordings**

<img width="290" alt="SCR-20231005-txgc"
src="https://github.com/MetaMask/metamask-extension/assets/46655/5e8dbbd5-45dc-4eda-999a-117580569190">


## **Related issues**

Fixes: #21220

This issue also references the position of the Account Picker but that
is addressed in
#21220

## **Pre-merge author checklist**

- [ ] I’ve followed [MetaMask Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've clearly explained:
  - [ ] What problem this PR is solving.
  - [ ] How this problem was solved.
  - [ ] How reviewers can test my changes.
- [ ] I’ve indicated what issue this PR is linked to: Fixes #???
- [ ] I’ve included tests if applicable.
- [ ] I’ve documented any added code.
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/LABELING_GUIDELINES.md)).
- [ ] I’ve properly set the pull request status:
  - [ ] In case it's not yet "ready for review", I've set it to "draft".
- [ ] In case it's "ready for review", I've changed it from "draft" to
"non-draft".

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

ui/components/multichain/global-menu/global-menu.js

@@ -13,6 +13,7 @@ import {
 import { lockMetamask } from '../../../store/actions';
 import { useI18nContext } from '../../../hooks/useI18nContext';
 import {
+  Box,
   IconName,
   ///: BEGIN:ONLY_INCLUDE_IN(snaps)
   Text,
@@ -54,6 +55,8 @@ import {
 import {
   AlignItems,
   BackgroundColor,
+  BlockSize,
+  BorderColor,
   Display,
   JustifyContent,
   TextAlign,
@@ -109,6 +112,11 @@ export const GlobalMenu = ({ closeMenu, anchorElement }) => {
         closeMenu={closeMenu}
         address={account.address}
       />
+      <Box
+        borderColor={BorderColor.borderMuted}
+        width={BlockSize.Full}
+        style={{ height: '1px', borderBottomWidth: 0 }}
+      ></Box>
       <MenuItem
         iconName={IconName.Connect}
         disabled={hasUnapprovedTransactions}