Line plot module: control symbol size

### Feature description

stakeholders report that it is difficult to easily see the difference between the line plot lines when using a splitting variable. there are number of reasons for this. here are three ideas to help address. suggested order is hopefully simplest to more complicated. happy to hop on a call to discuss further and goshawk function would need updated but didn't open and issue in that package.

1. Lines within a treatment arm are the same color and the same type. here's an example from the sample app filtering to only one treatment group and choosing SEX as the line splitting variable

Feature: would it be straight forward to include a UI element like what is in boxplot to increase the size of the line plot symbols?

2. Another idea to help differentiate within the line splitting context was to be able to set different line color within a treatment group. currently the line color and type selector functions within a treatment group.

Feature: I fear this would be quite complicated to add a nested line color and type selector within the treatment group value to control the split line aesthetics.

3. Another idea was to have the line type set automatically when a splitting variable is chosen. currently the line type is set to solid but this can be changed manually. however we observed when setting different line type it doesn't look like that is reflected in the legend. maybe the legend symbol/line is not big enough to show the line type.

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct.

### Contribution Guidelines

- [X] I agree to follow this project's Contribution Guidelines.

### Security Policy

- [X] I agree to follow this project's Security Policy.