Breakpoints can be set up in invalid places😱

## Environment data
`dotnet --info` output:
VS Code version: **_1.67.2_**
C# Extension version: **_1.25.0_**

## Steps to reproduce

1. Open a C# project **_.cs_** file.
2. Set up some breakpoints on comments, or blank lines.

## Expected behavior
Should not allow to set up breakpoints in invalid places.

## Actual behavior
Those invalid breakpoints are set. Even they get moved to the next closest valid places after starting to debug, and get restored after stopping debugging. It's very annoying.