Support .svelte

[jimmy-zhening-luo]

Language support:

• SvelteJS
• SvelteTS

Design investigations:

• Minimal use of prettier where possible
• For consistency, VSCode uses ESLint for .svelte file linting as well, rather than the Svelte for VSCode extension's built-in Prettier-based linting
• Where both Prettier and ESLint are needed if that's the case, ensure no conflict between Prettier and ESLint rules both in npm lint script & VSCode code action. Assume that Prettier will run first (during development on each save) and that ESLint would run and would align files with Prettier if not already aligned, and would only change the file in such a way that re-opening the file in the editor, adding a space e.g., and hitting save should result in 0 delta in the post-ESLint file.

[jimmy-zhening-luo]

done, v8