Shortcut to create link does not work properly (⌘-shift-a)

The shortcut for creating links in the editor component (⌘-shift-a) is unreliable doesn't work in Firefox(and other browsers) on Windows and Linux:

Test results on multiple browsers and OSs below

## Linux and Windows
Browser: Firefox

### Steps to reproduce
1. Select text.
2. Press Meta(Super) + Shift + A

### Expected result
Link creation dialog

### Actual result
Nothing

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## Windows
Browser: Edge

### Steps to reproduce
1. Select text.
2. Press Meta(Super) + Shift + A

### Expected result
Link creation dialog

### Actual result
Types uppercase A

-----------------------------------------------------
## Linux and Windows
Browser: Chromium and Chrome

### Steps to reproduce
1. Select text.
2. Press Meta(Super) + Shift + A

### Expected result
Link creation dialog

###Actual result
Link creation dialog

----------------------------------------------------
## Linux
Browser: Epiphany

### Steps to reproduce
1. Select text.
2. Press Meta(Super) + Shift + A

### Expected result
Link creation dialog

### Actual result
Types uppercase A

----------------------------------------------------

A good keybinding for the link creation dialog would be either:

- Ctrl + K like WP-Admin
- Alt + Shift + a / Control + Option + a like Calypso

If someone can test on Mac OS, it would be great :)

Start in the editor component's keyboard-shortcut handling and reproduce the selected-text shortcut on Firefox, Edge, Epiphany, Chromium, and Chrome across Linux and Windows. Compare the failing behavior with Chromium's working behavior, then verify that the chosen shortcut opens the link creation dialog instead of doing nothing or typing an uppercase A.

Written by the indexing model from the issue text.