[IndexTable] Align IndexTable checkbox header with other headers

[lhoffbeck]

WHY are these changes introduced?

Fixes #3989

On the IndexTable component, the select all checkbox isn't vertically centered with the rest of the the table headers. This is due to the checkbox having a display: inline-flex;, which adds whitespace since it's an inline element. This stackoverflow answer does a good job summarizing the issue.

WHAT is this pull request doing?

This PR adds a wrapper around the checkbox header to fix the alignment.

👀 BEFORE 👀

IndexTable / Simple Index Table example

IndexTable / Simple Index Table example w/sticky headers

👀 AFTER 👀
IndexTable / Simple Index Table example w/sticky headers

IndexTable / Simple Index Table example

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide
• For visual design changes, pinged one of @ HYPD, @ mirualves, @ sarahill, or @ ry5n to update the Polaris UI kit

[github-actions]

🟢 This pull request modifies 3 files and might impact 1 other files.

Details:

All files potentially affected (total: 1)

• /src/components/IndexTable/IndexTable.tsx

📄 UNRELEASED.md (total: 0)

Files potentially affected (total: 0)

🎨 src/components/IndexTable/IndexTable.scss (total: 1)

Files potentially affected (total: 1)

• /src/components/IndexTable/IndexTable.tsx

🧩 src/components/IndexTable/IndexTable.tsx (total: 0)

Files potentially affected (total: 0)

[AndrewMusgrave]

Love it 💯 I didn't previously notice it was off, but now I see the screenshot it's all I see 😂 I just approved percy so you're fully green now