Makes codeblock language and filename extensions consistent

[manovotny]

There was mismatched usage of js, jsx, ts, and tsx. Just tried to get them all in sync.

[delbaoliveira]

My preferences (though I don't feel strongly). Reasons:

• For most of the codebases I've seen, people rarely use the .jsx extension even for React component files.
• We have to use ```jsx annotation in code blocks to properly highlight JSX code, even if we use the .js extension.
• We have to use .tsx extension for files that contain JSX because TypeScript will throw errors for JSX code inside .ts files.

	Annotation	Extension
JavaScript files with JSX code	```jsx	.js
JavaScript files without JSX	```js	.js
TypeScript files with JSX	```tsx	.tsx
TypeScript files without JSX	```ts	.ts

[manovotny]

@delbaoliveira I updated everything according to your table and also added your table to the contribution guide.

[styfle]

Can we automate this with a lint rule?

Otherwise the mismatch will happen again in the future.