Add GIF and JPEG (JFIF) specs (#1237)

This adds the GIF and JPEG specifications to the list, as requested in #1089.

For GIF, the most canonical URL is the specification published under `w3.org`,
consistent with Specref and Wikipedia.

The notion of group does not mean much for the GIF spec. Also, the link to the
CompuServe website seems a bit wrong, but a URL is needed...

For JPEG, the usual reference is to the interchange format (JFIF). A PDF
version of the 1992 spec exists under `w3.org` at:
https://www.w3.org/Graphics/JPEG/jfif3.pdf

That is the spec that Specref returns for JPEG. That said, JPEG is published
and maintained by ISO/IEC under the name `iso10918-5`:
https://www.iso.org/standard/54989.html

That spec (and name) is used in the list instead of the PDF version because it
provides a more official entry point. If we want to record the public version
from 1992 somewhere, a new "JPEG" or "JPEG-1992" entry could perhaps be
created.

Schema and logic adjusted to allow ".txt" filenames (should not break anything)
for the GIF entry.

schema/definitions.json

@@ -10,7 +10,7 @@
 
     "filename": {
       "type": "string",
-      "pattern": "^[\\w\\-\\.]+\\.(html|pdf)$"
+      "pattern": "^[\\w\\-\\.]+\\.(html|pdf|txt)$"
     },
 
     "relativePath": {

specs.json

@@ -682,6 +682,11 @@
   "https://wicg.github.io/webpackage/loading.html",
   "https://wicg.github.io/webusb/",
   "https://wicg.github.io/window-controls-overlay/",
+  {
+    "url": "https://www.iso.org/standard/54989.html",
+    "shortname": "iso10918-5",
+    "shortTitle": "JPEG"
+  },
   {
     "url": "https://www.iso.org/standard/85253.html",
     "shortname": "iso18181-2",
@@ -775,6 +780,18 @@
       "repository": "https://github.com/w3c/w3process/"
     }
   },
+  {
+    "url": "https://www.w3.org/Graphics/GIF/spec-gif89a.txt",
+    "shortname": "GIF",
+    "shortTitle": "GIF",
+    "organization": "CompuServe Incorporated",
+    "groups": [
+      {
+        "name": "CompuServe Incorporated",
+        "url": "https://www.compuserve.com/"
+      }
+    ]
+  },
   "https://www.w3.org/TR/accelerometer/",
   "https://www.w3.org/TR/accname-1.2/",
   "https://www.w3.org/TR/ambient-light/",

src/determine-filename.js

@@ -11,7 +11,7 @@
 
 module.exports = async function (url) {
   // Extract filename directly from the URL when possible
-  const match = url.match(/\/([^/]+\.(html|pdf))$/);
+  const match = url.match(/\/([^/]+\.(html|pdf|txt))$/);
   if (match) {
     return match[1];
   }