Add keyboard shortcut for opening element blocker

[Brave-Matt]

Description

As it is right now in Brave on desktop, the "block element" option is only available through the context menu:

We should implement a keyboard shortcut that opens/starts the element blocker directly without need to right-click on the screen. This keystroke should also be customizable/added to the list of customizable commands in brave://settings/system/shortcuts.

Steps to reproduce

N/A

Actual result

N/A

Expected result

N/A

Reproduces how often

Easily reproduced

Brave version (brave://version info)

v1.78.102

Channel information

• release (stable)
• beta
• nightly

Reproducibility

• with Brave Shields disabled
• with Brave Rewards disabled
• in the latest version of Chrome

Miscellaneous information

No response