Autoformat file on save

[spotlesscoder]

Take the following file

version: '3.8'

services: 
  elasticsearch:
    image: docker.elastic.co/elasticsearch
    ports: 
      - 9200:9200
    - 9300:9300

You see the indentation is incorrect. When I save the file it should automatically correct the indentation

[bwateratmsft]

Another good suggestion, thanks! I've added this one to #2539 as well.

[PavelSosin-320]

@bwateratmsft Before going on this adventure please, check:

1. Formater and Linter use the same tokenizer and it is 100% reliable. Token list must be stable!
2. Git: ignore whitespace is used overall.
3. File is marked as modified after formatting, otherwise save may do nothing. Save after Linter warnings is possible or with approval, at least. Otherwise, if formatting can't be accomplished due to incomplete code changes the user will be stuck.
4. Good luck!

[PavelSosin-320]

@bwateratmsft Sorry for returning to this issue!
The LSP assumes that the file is formatted by the Language server upon receiving the notification willSsave and the server must return all "edits" to the client. It turns the state of the file to "modified". If If the language server confirms server capability The linter and the formatter share the tokenizer and the tokens list. So, this PR should be redirected to the Language Server implementation. The client must send willSave, didSave. and process willSave response. Hopefully, VSCode does it.

[bwateratmsft]

That's the thing--right now, there is no language server for compose. The YAML extension provides a language server for YAML more generally, and probably supports formatting, but not for Compose specifically. We're probably gonna end up making a language server for compose, which is what #2539 is largely about.

[PavelSosin-320]

Yes, I think there is no alternative. EGamma will be very disappointed if it will be not done. But also shame to development teams that proposed languages without publishing their Language formal gramars in the BNF or ANTLR forms.