Add metadata about the ECMAScript edition that each feature was introduced in

It's pretty common for test runners such as test262.fyi to have a way to filter tests per ECMAScript edition, and some people like to analyze the conformance of engines per ES version.

Currently some runners have opted for using "features.txt" to extract this information, since features have a clear edition where they were introduced, and this could be used to know which minimum edition a test supports. However, there's some duplicate effort around this, since all runners have to maintain a separate mapping from the feature to the corresponding edition that introduced it:

• boa_tester
• test262.fyi

Seeing this, it would be nice to remove this duplicate effort by moving this metadata here.

Possible points of discussion

• Unified file (features list + metadata) vs two files (features file + metadata file).
• File representation
  • Keep using .txt
  • Transition to yaml/json/toml
  • Some other format?
• Edition representation
  • Numeric (ES6 -> 6, ES12 -> 12, ...)
  • String (ES6 -> "ES6", ES12 -> "ES12", ...)
  • Other
• Sentinel value for features that haven't been published
  • Big number (255/99/65535)
  • Negative number (-1)
  • Some string? ("N/A", "ESNext", "undefined", "None", ...)

Start by reviewing the current features.txt representation and the edition mappings in boa_tester and test262.fyi linked in the issue. Resolve the open choices about file format, edition representation, and unpublished-feature values with maintainers before making changes. Done means the suite stores edition metadata once in an agreed format and downstream runners can use it.