29578 Rework PDF load handlers

[carolinepaulic]

Description

Trying again after my previous PR: https://github.com/allenai/pdf-component-library/pull/41

Changes in this PR:

• Added catch and finally blocks to onPdfLoadSuccess handler
  • catch accounts for potential errors thrown by getFirstPage
  • finally sets isLoading to false regardless of whether an error is thrown or not
• Updated scale's computePageDimensions to accept an IPDFPageProxy instead of individual properties from a PDFPageProxy as it was previously doing.
  • Created IPDFPageProxy to make unit testing easier
• Removed PdfPageSizeData and Point types that are no longer used

Testing Plan

• Hardcoded an invalid page number in getFirstPage to ensure the catch and finally blocks are working as intended
• Ran all unit tests
• Checked that PDF still loads as expected