use startupOptions#has function

[GustavoCaso]

This is my first time contributing to the project.

I was impresse of how powerful and rich the charm ecosystem is. I used for one of my toy projects https://github.com/GustavoCaso/expensetrace/tree/main/internal/cli/tui

I decided to have a look at the code of bubbletea to understand better how it works internally. I noticed that is some parts of the code it uses p.startupOptions.has and in others uses p.startupOptions&.

I thought it would be a good first contribution to update the code to use has in all cases that is checking p.startupOptions

Look forward for any feedback 😉