schema: Allow optional implementation_url

This change updates the compat-data schema and docs to allow per-browser
optional implementation_url values to be specified in the support data
for any browser; detailed description:

> An optional changeset URL or commit URL for the change which implemented
> the feature in the source code for the associated browser; e.g. a
> https://trac.webkit.org/changeset/ URL for a changeset whose subject
> line is in the form *"Enable [feature name/description]."* or *"Enable
> [feature name/description] by default."*. The presence of an
> `implementation_url` value indicates that the associated browser has
> spec-conformant support for the feature in its source code. Therefore,
> an `implementation_url` value can be added even if the feature hasn't
> yet shipped in a stable release, but instead only in a Nightly, Canary,
> or Technology Preview release — or even if the feature hasn't yet
> shipped in any release at all, but has only landed in the source code
> for the associated browser.

mdn#6896 (comment)
and mdn#6896 (comment)
are where the idea for this change arose. In summary, the rationale is to
have a record in BCD of what changeset landed support for a feature in a
particular browser’s codebase. Among other use cases, we have downstream
consumers who care only whether or not there’s an implementation of the
feature in a particular browser engine — even if that implementation
hasn’t yet actually shipped in a major release.

schemas/compat-data-schema.md

@@ -369,6 +369,18 @@ Example for two flags required:
 }
 ```
 
+#### `implementation_url`
+
+An optional changeset URL or commit URL for the change which implemented the feature in the
+source code for the associated browser; e.g. a https://trac.webkit.org/changeset/ URL for a
+changeset whose subject line is in the form _"Enable [feature name/description]."_ or
+_"Enable [feature name/description] by default."_. The presence of an `implementation_url`
+value indicates that the associated browser has spec-conformant support for the feature in
+its source code. Therefore, an `implementation_url` value can be added even if the feature
+hasn't yet shipped in a stable release, but instead only in a Nightly, Canary, or Technology
+Preview release — or even if the feature hasn't yet shipped in any release at all, but has
+only landed in the source code for the associated browser.
+
 #### `partial_implementation`
 
 A `boolean` value indicating whether or not the implementation of the sub-feature

schemas/compat-data.schema.json

@@ -39,6 +39,12 @@
             "required": ["type", "name"]
           }
         },
+        "implementation_url": {
+          "type": "string",
+          "format": "uri",
+          "pattern": "^http(s)?:\/\/[^#]+#.+",
+          "description": "An optional changeset URL or commit URL for the change which implemented the feature in the source code for the associated browser."
+        },
         "partial_implementation": {
           "type": "boolean",
           "description": "A boolean value indicating whether or not the implementation of the sub-feature deviates from the specification in a way that may cause compatibility problems. It defaults to false (no interoperability problems expected). If set to true, it is recommended that you add a note explaining how it diverges from the standard (such as that it implements an old version of the standard, for example)."

types.d.ts

@@ -195,6 +195,13 @@ export interface SimpleSupportStatement {
      */
     value_to_set?: string;
   }[];
+
+  /**
+   * An optional changeset URL or commit URL for the change which implemented the feature in the
+   * source code for the associated browser; e.g. a https://trac.webkit.org/changeset/ URL.
+   */
+  implementation_url?: string;
+
   /**
    * A `boolean` value indicating whether or not the implementation of the sub-feature follows
    * the current specification closely enough to not create major interoperability problems.