Add demonstration of tilde-comma bug

[backspace]

There’s a strange bug where a block with two selectors with at least one tilde causes syntax errors in some circumstances.

```css p ~ .something, h1 ~ .something { display: none; } ```	`CssSyntaxError: :4:31: Unclosed bracket`
```css p ~ .something, h1 { display: none; } ```	☑️
```css p, h1 ~ .something { display: none; } ```	☑️
```css p ~ .something, h3, h1 ~ .something { display: none; } ```	☑️
```css p + .something, h2 ~ .x, h1 + .something { display: none; } ```	`CssSyntaxError: :5:10: Unclosed bracket`

Mysterious… I have also seen Unknown word and Missed semicolon errors.

Stacktrace selection:

Module build failed (from ../../../../../../../../../Users/b/Documents/Cardstack/Code/glimmer-scoped-css/glimmer-scoped-css/dist/virtual-loader.js):
CssSyntaxError: <css input>:5:10: Unclosed bracket
    at Input.error (/Users/b/Documents/Cardstack/Code/glimmer-scoped-css/node_modules/.pnpm/postcss@8.4.21/node_modules/postcss/lib/input.js:148:16)
    at Parser.unclosedBracket (/Users/b/Documents/Cardstack/Code/glimmer-scoped-css/node_modules/.pnpm/postcss@8.4.21/node_modules/postcss/lib/parser.js:532:22)

It’s not clear to me where the problem actually is, is it actually within this library?

[backspace]

Fixed in #12.