Lint test `features` tag (#1060)

A recent commit introduced a document that enumerated acceptable values
for the test "features" metadata tag. However, this list was incomplete,
and maintaining it placed extra burden on the project owners.

Restructure the document into a machine-readable format. Add entries for
all previously-omitted values. Add in-line documentation with
recommendations for maintenance of the file. Extend the project's
linting tool to validate tests according to the document's contents.

1 files changed, 99 insertions, 0 deletions

diff --git a/features.txt b/features.txt
new file mode 100644
index 000000000..41323de4e
--- /dev/null
+++ b/features.txt
@@ -0,0 +1,99 @@
+# Proposed language features
+#
+# This project accepts tests for language proposals that have reached stage 3
+# in TC39's standardization process. Those tests should be annotated with a
+# dedicated feature flag so that consumers may more easily omit them as
+# necessary.
+#
+# https://github.com/tc39/process-document
+
+# Async Iteration and Generators
+# https://github.com/tc39/proposal-async-iteration
+async-iteration
+Symbol.asyncIterator
+
+# Object rest/spread properties
+# https://github.com/tc39/proposal-object-rest-spread
+object-rest
+object-spread
+
+# RegExp s (dotAll) flag
+# https://github.com/tc39/proposal-regexp-dotall-flag
+regexp-dotall
+
+# RegExp lookBehind
+# https://github.com/tc39/proposal-regexp-lookbehind
+regexp-lookbehind
+
+# RegExp named groups capturing
+# https://github.com/tc39/proposal-regexp-named-groups
+regexp-named-groups
+
+# RegExp Unicode Property Escapes
+# https://github.com/tc39/proposal-regexp-unicode-property-escapes
+regexp-unicode-property-escapes
+
+# Shared Memory and atomics
+# https://github.com/tc39/ecmascript_sharedmem
+SharedArrayBuffer
+
+# Standard language features
+#
+# Language features that have been included in a published version of the
+# ECMA-262 specification. These flags are largely maintained for historical
+# reasons, though their use for relatively new features (i.e. prior to
+# availability across major implementations) is appreciated.
+
+ArrayBuffer
+Array.prototype.values
+arrow-function
+async-functions
+caller
+class
+const
+DataView
+DataView.prototype.getFloat32
+DataView.prototype.getFloat64
+DataView.prototype.getInt16
+DataView.prototype.getInt32
+DataView.prototype.getInt8
+DataView.prototype.getUint16
+DataView.prototype.getUint32
+DataView.prototype.setUint8
+default-arg
+default-parameters
+destructuring-binding
+Float64Array
+generator
+generators
+Int8Array
+let
+Map
+new.target
+Proxy
+Reflect
+Reflect.construct
+Reflect.set
+Reflect.setPrototypeOf
+Set
+String#endsWith
+String#includes
+super
+Symbol
+Symbol.hasInstance
+Symbol.isConcatSpreadable
+Symbol.iterator
+Symbol.match
+Symbol.replace
+Symbol.search
+Symbol.species
+Symbol.split
+Symbol.toPrimitive
+Symbol.toStringTag
+Symbol.unscopables
+tail-call-optimization
+template
+TypedArray
+Uint8Array
+WeakMap
+WeakSet