prettier: support read code from stdin and output to stdout

[axetroy]

closes #457

Try it out

run in Linux/MacOS

curl https://raw.githubusercontent.com/denoland/deno_std/master/prettier/testdata/0.ts | deno --reload https://github.com/axetroy/deno_std/raw/prettier_stdin/prettier/main.ts

run in Windows

(iwr https://raw.githubusercontent.com/denoland/deno_std/master/prettier/testdata/0.ts).Content | deno --reload https://github.com/axetroy/deno_std/raw/prettier_stdin/prettier/main.ts

Design goals:

curl https://example.com/test.ts | deno fmt
curl https://example.com/test.js | deno fmt --stdin-parser=babel
curl https://example.com/test.json | deno fmt --stdin-parser=json
curl https://example.com/test.md | deno fmt --stdin-parser=markdown

What it changes?

1. add --stdin flag to read the code from stdin. only use in the test.
2. add --stdin-parser to set parser. tell prettier what code it is from stdin. default: typescript
3. catch error and output to stderr instead of stdout

Why I need this feature

I am writing a formatter for justjavac.vscode-deno.

currently, deno fmt is a file-based formatting tool.

But in the IDE/editor not based on files, it may be a temporary string (if the file is modified, but the file is not maintained).

So this PR is for better-integrated development tools.

[axetroy]

/cc @ry it's ready for review.

[kt3k]

I think we should document --stdin-parser and --stdin options in Options: section of help message of prettier/main.ts

[axetroy]

I think we should document --stdin-parser and --stdin options in Options: section of help message of prettier/main.ts

Can you help me see if the documentation for --stdin-parser and --stdin is correct?

[kt3k]

The help message looks good to me.

[kt3k]

LGTM

[ry]

LGTM