Add API for getting and setting the progress

[hirasso]

Description

Add an API for getting and setting the scroll progress:

// getter
const progress = mirror.progress;
// setter
mirror.progress = {x: 0, y: 0.5};
// ...or only change one direction
mirror.progress = {y: 0.5};
// ...or, for both directions at once
mirror.progress = 0.5;

Checks

• The PR is submitted to the main branch
• The code was formatted before pushing (npm run format)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

Additional information

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