feat: picker-based input method for abbreviations in non-editor contexts

[mhuisi]

This PR adds three new abbreviation-related commands that have the goal of making it easier to insert Unicode symbols into non-editor dialogs and to make abbreviations more discoverable:

• 'Find Unicode Symbol...': Displays a picker dialog that allows searching for Unicode symbols either by abbreviation or by symbol. Selecting an entry then provides an option to either copy the symbol or to insert it into the currently active text editor. Displayed in the forall menu.
• 'Copy Unicode Symbol...': Like 'Find Unicode Symbol', but selecting a symbol from the dialog immediately copies it instead of opening another dialog to choose whether the symbol should be inserted or copied. Bound to Ctrl+Alt+\ when no text editor is focused.
• 'Insert Unicode Symbol...': Like 'Find Unicode Symbol', but selecting a symbol from the dialog immediately inserts it instead of opening another dialog to choose whether the symbol should be inserted or copied. Bound to Ctrl+Alt+\ when a text editor is focused.