Edit: Starting from 1.52 it’s possible from keybindings GUI:

What stops you from editing keybindings.json?
{
"key": "left",
"command": "cursorLeft",
"when": "textInputFocus"
},
{
"key": "alt+a",
"command": "cursorLeft"
}
It can be opened from Command Palette Preferences: Open Keyboard Shortcuts (JSON)
Or by clicking the file icon from keybindings GUI page:
