[svelte2tsx] Add comments to blanked script/style tags for perf

[halfnelson]

jasonlyu09 from discord discovered Svelte's parse method is very slow when scripts contain lots of whitespace.

We assume it is something to do with a regex that cleans trailing spaces. (may be similar to this sindresorhus/trim-right#3)

Instead of changing the regex in svelte, we just stop making too much white space by introducing /**/ comments at the end of lines when blanking out the script tag.

Added a test.

With fix:

Without fix:

[dummdidumm]

👍
FYI @jasonlyu123