Make formatting a built-in feature (avoid depending on Terraform CLI)

[radeksimko]

Context

Throughout past years we received numerous reports about "not working formatting". With a little bit of eye twitching, they can be roughly divided in two buckets:

• VS Code related problems to either settings or extension conflicts
• Terraform CLI related problems
  • missing or misconfigured entry in $PATH
  • Terraform not installed at all
  • Terraform "hidden" behind wrappers such as tfenv and other version managers, which return unexpected output from terraform version when the given TF version isn't already installed

This issue focuses on the second bucket which is more within our control.

Past related issues:

• Please support tfenv vscode-terraform#1435
• format not working vscode-terraform#1431
• Document Terraform (CLI) as a dependency vscode-terraform#552
• Autoformatting the file on save does not work. vscode-terraform#1426
• auto-format not working on mac vscode-terraform#1341
• Format on save does not work vscode-terraform#1265
• There is no formatter for 'terraform' files installed. vscode-terraform#617
• Cannot get autoformatting (format on save) working - VS Code vscode-terraform#542

This also happens to be a pre-requisite for #613

Proposal

Consider re-implementing terraform fmt as part of the language server, to reduce the "number of moving parts" and make formatting work out of the box for users.