Add 'merge cell above/below' commands with shortcuts

[krassowska]

References

Fixes #6330

Code changes

Added mergeAbove parameter in mergeCells function with default value false. When false this function will merge with a cell below, when true it will merge with a cell above.

User-facing changes

Added "Merge cell above" and "Merge cell below" commands with keyboard shortcuts proposed in #6330 (Ctrl+Backspace for cell above and Ctrl+Shift+M for cell below). The options are displayed in the main menu and context menu (not sure if it should stay here as it's getting long).
"Merge selected cells" works the same.

Backwards-incompatible changes

I don't know of any.

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[jtpio]

Thanks @krassowska! Do you mind adding a test case to cover the new mergeAbove parameter?

jupyterlab/packages/notebook/test/actions.spec.ts

Line 200 in 0e23100

describe('#mergeCells', () => {

[krassowska]

Thank you @jtpio. I've added two tests. All tests are passing on my computer, but some other things are now failing on GitHub and I don't understand why.

[blink1073]

Thank you!

[blink1073]

Kicking CI

[blink1073]

Update after #10094

Good catch!

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