Function isSolidLine

isSolidLine(value: unknown): value is SolidLine<FillStyle>
SolidLine type guard.

Use to verify that an LineStyle is a SolidLine.

Example:
```
line.setStrokeStyle((stroke) => isSolidLine(stroke) ? stroke.setThickness(10) : new SolidLine() )
```
Returns
Boolean indicating if value is a SolidLine
Parameters
- value: unknown
  
  LineStyle to check
Returns value is SolidLine<FillStyle>