Added JSON-based changes to auto-generate the changelog.

Solves merge conflicts between changelogs, by using sorting and
type-based merging for the changelog from a series of JSON files. This
allows conflict-free generation of a changelog using JSON-based changes,
stored in `.changes`.

Each entry can be object or array-based: the array must contain objects,
where each object is a different change. The entries are added to the
changelog, in sorted order, and the validity of the changelog is
verified.

To ensure fidelity of the changelog changes, the `no changelog` flag or
at least 1 change (either removed, or added/dified and almovidated) for
a JSON file under `.changes` must occur.

A `pre-release-hook` has been added so all these changes are automerged
into the CHANGELOG prior to release.

.changes/918.json

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+{
+    "description": "use JSON-based files to autogenerate CHANGELOG.md",
+    "issues": [662],
+    "type": "internal"
+}

.changes/README.md

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+# Changes
+
+This stores changes to automatically generate the changelog, to avoid merge conflicts. Files should be in a JSON format, with the following format:
+
+```json
+{
+    "description": "single-line description to add to the CHANGELOG.",
+    "issues": [894],
+    "type": "added",
+    "breaking": false
+}
+```
+
+Valid types are:
+- added (Added)
+- changed (Changed)
+- fixed (Fixed)
+- removed (Removed)
+- internal (Internal)
+
+`breaking` is optional and defaults to false. if `breaking` is present for any active changes, a `BREAKING:` notice will be added at the start of the entry. `issues` is also optional, and is currently unused, and is an array of issues fixed by the PR, and defaults to an empty array.
+
+The file numbers should be `${pr}.json`. The `pr` is optional, and if not, an issue number should be used, in the `_${issue}.json` format. We also support multiple PRs per entry, using the `${pr1}-${pr2}-(...).json` format.
+
+If multiple changes are made in a single PR, you can also pass an array of entries:
+
+```json
+[
+    {
+        "description": "this is one added entry.",
+        "issues": [630],
+        "type": "added"
+    },
+    {
+        "description": "this is another added entry.",
+        "issues": [642],
+        "type": "added"
+    },
+    {
+        "description": "this is a fixed entry that has no attached issue.",
+        "type": "fixed"
+    }
+]
+```
+
+See [template](/.changes/template) for sample object and array-based changes.

.changes/template/940.json

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+[
+    {
+        "description": "this is one added entry.",
+        "issues": [630],
+        "type": "added"
+    },
+    {
+        "description": "this is another added entry.",
+        "issues": [642],
+        "type": "added"
+    },
+    {
+        "description": "this is a fixed entry that has no attached issue.",
+        "type": "fixed"
+    },
+    {
+        "description": "this is a breaking change.",
+        "issues": [679],
+        "breaking": true,
+        "type": "changed"
+    }
+]

.changes/template/978.json

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+{
+    "description": "sample description for a PR adding one CHANGELOG entry.",
+    "issues": [437],
+    "type": "fixed"
+}

.changes/template/979-981.json

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+{
+    "description": "this has 2 PRs associated.",
+    "issues": [441],
+    "type": "added"
+}