searchEditor: Add option to peek with a single click

[mkasenberg]

The option
"search.searchEditor.singleClickBehaviour": "peekDefinition" will allow to open a Peek Window in Search Editor with a single click.

I really need this to make code browsing easier when I'm in "snack mode" 🍩 ☕

Fixes #206542

[mkasenberg]

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[andreamah]

Looks good! Thanks and happy snack-mode-ing! ☕