Inaccurate breakpoints in VS Code during debugging

Describe the bug Breakpoints are inaccurate in VS Code. Sometimes they take you to transpiled code by Svelte/TypeScript. Other times they take you just after the breakpoint line. They also may not be hit at all. It seems related to source mapping.

To Reproduce Here is a repro link

Try setting breakpoints at lines 4 to 9 in App.svelte.

Expected behavior A breakpoint should be consistently hit just before the line is executed anywhere in the script parts of svelte files.

Information about your Svelte project:

System:
    OS: macOS 11.1
    CPU: (4) x64 Intel(R) Core(TM) i7-4558U CPU @ 2.80GHz
    Memory: 6.70 GB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 15.5.0 - /usr/local/bin/node
    yarn: 1.22.10 - /usr/local/bin/yarn
  Browsers:
    Chrome: 87.0.4280.88
    Firefox: 83.0
    Safari: 14.0.2
  npmPackages:
    rollup: ^2.35.1 => 2.35.1 
    svelte: ^3.31.1 => 3.31.1
  IDE: VS Code Insider v1.53.0-insider (with Svelte for VS Code v102.8.0) and also JetBrains Rider 2020.3

Severity It is annoying not being able to debug and affects productivity.

[dummdidumm]

Could you try again with latest versions of svelte, svelte-preprocess and rollup-plugin-svelte? This might work as expected now.

[dummdidumm]

🎉