Revisit code formatting for `/packages/lit-dev-content/samples/`

[louis-bompart]

There are a couple of code formatting issues in the cheat sheet. (e.g. inconsistent spacing, missing newlines)

The packages/lit-dev-content/samples directory was prettier ignored in #560.

In a fix-twice fashion, let's try to revisit prettier so that added content is consistently formatted.

(discussed in lit eng. meeting on April 10th, I'll work on it in the coming days)

[cseas]

Hey @louis-bompart, can I take it up if it's not being actively worked on?

[louis-bompart]

Hey @cseas , I tried some stuff on #1415, but did not have time to nag to validate whether the results were satisfactory.
If you want to propose an alternative approach, feel free to take it up 🙌