feat: implement auto format on save

[axetroy]

#10

What is the purpose of this pull request? (put an "X" next to item)

• Documentation update
• Bug fix (template)
• New feature(template)
• Add a config
• Add a command
• Add something to the plugin
• Other, please explain:

**What changes did you make? **

[axetroy]

deno 的格式化也是使用 prettier 的

使用相应的包 https://github.com/denoland/deno_std/blob/master/prettier/prettier.ts

如果没有下载的话，在第一次格式化的时候下载编译，可能会花很长时间。

所以，这里就需要确定下: 要不要自动下载且编译 prettier

[justjavac]

ahould we register languages.registerDocumentFormattingEditProvider and languages.registerDocumentRangeFormattingEditProvider instead of directly formatting it?

• https://code.visualstudio.com/api/references/vscode-api#languages

[axetroy]

yes. I will re-push soon

[axetroy]

languages.registerDocumentFormattingEditProvider is only for formatter.

If we want to implement automatic formatting when saving.

There are 2 ways:

1. listen on workspace.onDidSaveTextDocument and trigger format like this PR do above
2. set build-in setting Format On Save to true and set deno formatter to default. The bad is Format On Save effect all language

[axetroy]

It's not yet for merge.

deno xxx.ts --fmt will not print the result to stdout.

Therefore, there are still some problems in integration.

waiting for this denoland/std#332