Colorize Firestore Emulator Coverage

Context: we have pretty extensive Firestore security rules and when viewing the emulator coverage report, it gets really difficult to see what went wrong and where due to the sheer size of it.

I'm using the following CSS overrides to provide some visual cues:

```css
.coverage-expr:hover {
border-bottom: 3px dashed rgb(0, 0, 0);
cursor: default;
}

.coverage-expr[title="Expression never evaluated"] {
background: lightyellow;
}

.coverage-expr[title^="Value true returned"] {
background: lightgreen;
}

.coverage-expr[title^="Value false returned"] {
background: lightcoral;
}

.coverage-expr[title^="Expression short-circuited"] {
background: lightgrey;
}
```

Sample:



The CSS selectors might not be perfect, but we find them pretty helpful while analyzing coverage.

It would be great if something like this was incorportated into the emulator by default.