[#2084] Add attribute for TZIP-16 compatibility

Motivation and Context

In LSP we want to have diagnostics containing TZIP-16 warnings. Since those work on-the-fly, we need a way to track storages that the user considers TZIP-16 compatible.

The simplest way to do so is to add the dedicated attribute/decorator.

Pros:

• Easy to implement. If we compare this to options where "marked as TZIP-16" is remembered in memory - in those options the marker will be hard to track in the presence of code edits.
• Information about TZIP-16 compliant storages persists across coding session, so is more reliable as a testing tool.

Cons:

• Kinda pollutes the source code.
• Running all checks on IDE startup may take long since they involve downloading data (metadata can refer to external sources).
• At the moment, is not supported by the compiler, which may cause an impression of inconsistency in the ecosystem.

Description

So this MR makes LIGO compiler aware of the tzip16_compatible attribute and mentions it in the LIGO documentation.

We can consider different names for the attribute, e.g. tzip16_compliant may be better than the current variant.

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Added tzip16_compatible attribute for indicating that the given storage value is expected to satisfy TZIP-16 metadata rules.

This attribute does nothing on itself at the moment, but some IDEs may provide additional verification.

Example (CameLIGO):

type storage =
  {
   data : int;
   metadata : (string, bytes) big_map
  }

[@tzip16_compatible]
let storage_sample : storage =
  {
   data = 5;
   metadata =
     Big_map.literal
       [
         ("",
          [%bytes
          "https://ipfs.io/ipfs/QmSBc8QuynU7bArUGtjwCRhZUbJyZQArrczKnqM7hZPtfV"])
       ]
  }

Example (JsLIGO):

type storage = { data: int; metadata: big_map<string, bytes> }

@tzip16_compatible
const storage_sample: storage = {
  data: 5,
  metadata: Big_map.literal(
    list(
      [
        [
          "",
          (
            bytes
            `https://ipfs.io/ipfs/QmSBc8QuynU7bArUGtjwCRhZUbJyZQArrczKnqM7hZPtfV`
          )
        ]
      ]
    )
  )
}

Checklist:

• If a new syntax has been introduced, put a message on slack ligo-lsp
• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).