Adds twoslash to the gatsby side

[orta]

See: https://twitter.com/orta/status/1197228756370366465 because I didn't save the GIF - doh

[orta]

This PR turned into a bit more work than I expected, because this was the first time there was interlinking between deps I had to make sure they were all built beforehand.

[github-actions]

	Messages
📖	Deployed to a PR branch - tsconfig old handbook

Got false positives?

Make changes to the global settings spellcheck.json in /artsy/peril-settings.

Typos for README.md

Line	Typo
1	### gatsby-remark-twoslasher-code-blocks
3	up code blocks to run through ts-twoslasher:
32	t only works when you specify twoslash in your codeblock
32	you specify twoslash in your codeblock

Typos for downlevelIteration.md

Line	Typo
12	r / ofloop on any object is **downleveled** to a traditionalfor` loop:
23	% compliant with ECMAScript 6 behavior.
27	lementation (either native or polyfill).
51	escription, it sounds easy to downlevel to ES5:
59	l create an own property if spreaded, but will not if built using
73	more accurately emulate ES 6 behavior.

Typos for README.md

Line	Typo
1	# TypeScript TwoSlasher
4	by the [fourslash test system](https://github.c
13	ndling showing the results of transpilation with certain flags
21	- Think about how to ship to npm
24	The twoslash markup API lives inside your
45	Finally you can set any tsconfig compiler flag using this synt
442	ebug logs by running with the env var of DEBUG="*".
450	is optimized and bundled with Rollup into multiple formats (Common
450	Rollup into multiple formats (CommonJS, UMD, and ES Module).

Lighthouse Scores

/

Performance	Accessibility	Best Practices	SEO	Progressive Web App
0.95	0.9	0.93	0.7	1

/tsconfig

Performance	Accessibility	Best Practices	SEO	Progressive Web App
0.94	0.9	1	0.8	0.96

/docs/handbook/integrating-with-build-tools.html

Performance	Accessibility	Best Practices	SEO	Progressive Web App
0.97	0.9	1	0.77	0.96

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