docs: apply common 'styles.css' across all samples all langs; fix styles

[wardbell]

Per @naomiblack, all DevGuide samples must adhere to the "Tutorial: Tour of Heroes" CSS styles.

This PR captures the "Tutorial: Tour of Heroes" styles in the canonical ~/_examples/styles.css and applies that style to all devguide samples — TS, JS, and Dart.

add-example-boilerplate gulp task automatically adds the canonical ~/_examples/styles.css to all example projects including Dart projects. remove-example-boilerplate removes it.

A DevGuide example should never create or alter styles.css. styles.css is excluded from source control except for the canonical ~/_examples/styles.css

Todos:

• Update author's guide to reflect these instructions
• Tell authors about them and remind authors to call add-example-boilerplate