docs: add typescript version range decision (#1716)

decisions/typescript-versioning.md

@@ -0,0 +1,22 @@
+# TypeScript versioning
+
+## Strict TypeScript peer dependency version
+
+The `typescript` peer dependency version _must always have a strict upper range_ defined:
+
+```
+(lower)     (upper)
+>= 4.4.x <= 5.1.x
+```
+
+Fixing the upper range of TypeScript acts as an explicit upper boundary of the list of the supported TypeScript versions.
+
+> Context: TypeScript is [not distributed according to semver](https://github.com/microsoft/TypeScript/issues/14116). Instead, it's `{marketing}.{major}.{minor}` versioning pattern. This means that it may and does contain breaking changes across what consumers perceive as _minor_ versions. I've had numerous fights with this as it's not uncommon for TypeScript to exhibit different behavior across minor versions (the same types compile on 4.6, break on 4.7, then compile without issue on 4.8).
+
+## TypeScript version compliance
+
+Every version within the `typescript` peer dependency version range must have a corresponding TypeScript compliance test to guarantee the library's operatbility on that version.
+
+A compliance test is represented as a regular _typings test_ compiled using a specific version of TypeScript. You can learn more in the [typings tests](/test/typings).
+
+The typings test will automatically attempt to look up `tsconfig.{major}.{minor}.json` file corresponding to the currently installed version of TypeScript. The compliance is then achieved by installing different TypeScript versions from the supported range and running the existing typings tests on that version.