refactor(vitest): rename installQuasar in installQuasarPlugin

[ilmalte]

Renamed installQuasarPlugin in installQuasar in accordance with the interface exported by the package @quasar/quasar-app-extension-testing-unit-vitest

What kind of change does this PR introduce?

• [x] Documentation

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information: Seems like vitest export is different from Jest and Cypress. If there is no technical reason behind, this could be misleading 😄

[IlCallo]

Seems like the problem here is the other way around: the helper function should be named installQuasarPlugin to be coherent with Cypress and Jest implementation of the same helper, since we'll need to abstract it into a package shared by all testing harnesses in the future

Could you perform the change? I'll then bump deps and release a new version under the beta tag

[ilmalte]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] New test runner
• [ ] Documentation
• [ ] Feature
• [ ] Code style update
• [x] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [x] Yes
• [ ] No

If yes, please describe the impact and migration path for existing applications:

The vitest API after this PR will change. The export function will no more be installQuasar but installQuasarPlugin in order to be coherent with Cypress and Jest. I renamed it and introduced this breaking change, but we could still consider using a JSDoc comment to mark the old, deprecated code.

[IlCallo]

Being in alpha phase, I feel it's ok to add a breaking change at this stage without deprecation Would be worse if we did it in beta/stable phase

Thanks for contributing!