29578 Refactor PDF load handlers to use async/await

[carolinepaulic]

Description

This covers the async/await subtask of https://github.com/allenai/scholar/issues/29578

Changes in this PR:

• Updated getFirstPage to an async function
• Updated onPdfLoadSuccess anonymous callback function to async
• Updated onPdfLoadSuccess anonymous callback function to await the results of getFirstPage

Testing Plan

• Ran all unit tests
• Checked that PDF still loads as expected

[carolinepaulic]

Moving changes into a new branch and PR: https://github.com/allenai/pdf-component-library/pull/44