Linter: use async I/O for greater speed

[bershanskiy]

#16437 introduced a new linter which unfortunately uses only synchronous I/O: existsSync, existsSync (multiple times on the same file!), readFileSync, readdirSync.

browser-compat-data/test/lint.js

Line 34 in 2888cf5

if (!fs.existsSync(file)) {

browser-compat-data/test/lint.js

Line 39 in 2888cf5

if (fs.statSync(file).isFile() && path.extname(file) === '.json') {

browser-compat-data/test/lint.js

Line 47 in 2888cf5

const rawFileData = fs.readFileSync(file, 'utf-8').trim();

browser-compat-data/test/lint.js

Line 63 in 2888cf5

if (fs.statSync(file).isDirectory()) {

browser-compat-data/test/lint.js

Line 65 in 2888cf5

.readdirSync(file)

This makes linter very slow, on a decent laptop with an SSD time npm run lint produces:

real    0m8.531s
user    0m0.046s
sys     0m0.106s

I like async I/O a lot and if you would be interested in reviewing a PR for async I/O, I can create one.

[queengooborg]

The linter has actually not been asynchronous before! I'm totally down for making it asynchronous though to speed it up.