feat: add TextMate document partitioner

Author: sebthom

documentation/DOCUMENT_PARTITIONER.md

@@ -0,0 +1,52 @@
+# TM Partitioner (IDocumentPartitioner)
+
+- Partitioning ID: `tm4e.partitioning`
+- Partition type naming: `tm4e:<root-scope>` (e.g., `tm4e:text.html.basic`, `tm4e:source.js`, `tm4e:source.css`).
+- Installed automatically by `org.eclipse.tm4e.ui` for any document that resolves a TextMate grammar. It is added as a secondary partitioning and does not replace an editor’s default partitioner.
+
+How to consume from code
+
+- Get partitions from the TM4E partitioner via `IDocumentExtension3`:
+  - `var ext3 = (IDocumentExtension3) doc;`
+  - `var part = ext3.getDocumentPartitioner("tm4e.partitioning");`
+  - `ITypedRegion[] regions = part.computePartitioning(offset, length);`
+
+Example (content assist/feature code):
+
+```java
+IDocument doc = viewer.getDocument();
+if (doc instanceof IDocumentExtension3 ext3) {
+  var tmPart = ext3.getDocumentPartitioner("tm4e.partitioning");
+  if (tmPart != null) {
+    ITypedRegion r = tmPart.getPartition(caretOffset);
+    switch (r.getType()) {
+      case "tm4e:source.js":
+        // offer JavaScript proposals
+        break;
+      case "tm4e:source.css":
+        // offer CSS proposals
+        break;
+      default:
+        // base language behavior
+    }
+  }
+}
+```
+
+Generic Editor contributions
+
+- Register your feature with the Generic Editor as usual (e.g., content assist, hovers). Inside your implementation, use the code above to query TM4E’s partitioning and branch behavior based on `r.getType()`.
+- You do not need to install a partitioner yourself; `org.eclipse.tm4e.ui` installs it automatically when a grammar exists.
+
+Example plugin.xml (content assist):
+
+```xml
+<extension point="org.eclipse.ui.genericeditor.contentAssistProcessors">
+  <contentAssistProcessor
+      class="com.example.MyTM4EAwareAssist"
+      contentType="org.eclipse.core.runtime.text"
+      targetId="org.eclipse.ui.genericeditor.GenericEditor"/>
+  <!-- partitionType here refers to the editor's default partitioning;
+       to use TM4E partitions, query tm4e.partitioning from your code. -->
+  </extension>
+```

org.eclipse.tm4e.ui.tests/plugin.xml

@@ -15,6 +15,7 @@
    <extension
          point="org.eclipse.core.contenttype.contentTypes">
       <content-type
+            base-type="org.eclipse.core.runtime.text"
             file-extensions="ts"
             id="org.eclipse.tm4e.ui.tests.testContentType"
             name="Test Content Type for TM4E (typescipt)"