Switch GitHub highlighting to `hashicorp/syntax`

[radeksimko]

Background

Currently GitHub uses https://github.com/alexlouden/Terraform.tmLanguage for highlighting HCL:

https://github.com/github/linguist/blob/cddf7476af4c95d1572956ffc5c0cb84f7e431c5/lib/linguist/languages.yml#L2384-L2399

That project is primarily used for highlighting in Sublime Text and has also limited maintenance capacity as per https://github.com/alexlouden/Terraform.tmLanguage/issues/36

Proposal

• [x] https://github.com/hashicorp/syntax/pull/34
• [x] Release v0.1 of https://marketplace.visualstudio.com/items?itemName=hashicorp.hcl
• [x] Wait for a few weeks to gather any feedback from VS Code users
• [x] Bring over syntax tests from https://github.com/alexlouden/Terraform.tmLanguage (and remember to give credit to Pat Gaffney)
• [x] Ensure the configuration under test produces comparable or better highlighting to https://github.com/alexlouden/Terraform.tmLanguage

Raise PR to github/linguist to:

• [x] switch GitHub's highlighting of HCL to hcl grammar in this repo
• [ ] add GitHub's highlighting of Sentinel via grammar in this repo