feature: char mode highlight backdrop default value to false

[zhaoxuyll]

Did you check the docs?

• [X] I have read all the flash.nvim docs

Is your feature request related to a problem? Please describe.

Is it considered clearer to keep the same default values as the search pattern?

Describe the solution you'd like

Be consistent.

Describe alternatives you've considered

No

Additional context

No response

[folke]

feel free to change it in your own config