Minor fixes to Cypress e2e tests

[tdonohue]

Description

This small PR fixes two small issues with Cypress e2e tests:

• First, it re-enables an accessibility validation test in admin-sidebar.cy.ts, as the prior accessibility issue was fixed in Accessibility of the administration menu (submenus) on the left hand side  #2178.
• Second, it fixes the tsconfig.json for the Cypress tests to allow IDEs (especially VS Code) to recognize *.cy.ts files & provide autocomplete, etc.
  • This issue appeared to be related to Cypress 12.13.0 with Typescript 5.1 fails to compile cypress-io/cypress#26930 (comment)
  • The fix was to add ../node_modules to the (default) list of typeRoots that are defined in our root tsconfig.json file. That small addition allows IDEs to more easily locate the type definitions related to Cypress, allowing for autocomplete, etc.

Instructions for Reviewers

• The first change will be auto-tested by this PR. If the accessibility check succeeds, then this fix is correct.
• The second changes is most easily tested in VS Code by opening any *.cy.ts file in the cypress directory. Currently, on main, if you open one of these files, a lot of the type definitions will be unrecognized & you'll see red lines in the opened files. However, with this PR in place, those red lines go away.

NOTE: There are no direct changes to DSpace in this PR. The two small changes only impact e2e tests.

[tdonohue]

Merging this immediately as this doesn't modify any DSpace code, just tests & all tests pass.

Plus, the minor changes to the cypress tsconfig.json finally fix some annoying issues with editing our e2e tests in IDEs. If anyone finds issues later on, I'll gladly fix them up.