Add horizontalAlignment to Menu.open() options

[andrewfulton9]

Closes #709

Adds an "align" option to Menu.IOpenOptions that can be set to "left" or "right" and adjusts the menu position so the top left or right corner is matched to the given x and y coordinates based on the given align value. It defaults to left or right depending on the document dir attribute to account for the given language direction.

[krassowski]

Thank you @andrewfulton9!

1. To refresh review/api/widgets.api.md you can run yarn run api locally
2. The tests are failing on Mac/Safari due to rounding issues:

 ❌ @lumino/widgets > Menu > #open() > should accept align menu flags
      AssertionError: expected 'translate(205.53125px, 300px)' to equal 'translate(205.531px, 300px)'
      + expected - actual
      
      -translate(205.53125px, 300px)
      +translate(205.531px, 300px)
      
      at tests/lib/bundle.test.js:39356:59

 ❌ @lumino/widgets > Menu > #open() > align should default to right if language direction is rtl
      AssertionError: expected 'translate(205.53125px, 300px)' to equal 'translate(205.531px, 300px)'
      + expected - actual
      
      -translate(205.53125px, 300px)
      +translate(205.531px, 300px)   

Maybe we could use a regular expression pattern here, optionally allowing the number to have more decimal digits?

[andrewfulton9]

@krassowski, I updated the tests so only the whole number of the x value is checked. It should be good to go now!

[krassowski]

To make this more future-proof, should we call it horizontalAlignment instead? So that we can add verticalAlignment if we need it later?

Otherwise it looks great!

[andrewfulton9]

Good point! I just updated that.

[krassowski]

Thank you @andrewfulton9!