feat: indent on enter after starting a focus block (#328)

* feat: indent on enter after starting a focus block

* docs: create `language-configuration.md`
* remove corresponding comment from json

docs/language-configuration.md

@@ -4,6 +4,27 @@ See [the official VS code documentation](https://code.visualstudio.com/api/langu
 
 Section titles here are top-level fields in the JSON file, and link to their corresponding section in the official documentation.
 
+## [`onEnterRules`](https://code.visualstudio.com/api/language-extensions/language-configuration-guide#on-enter-rules)
+
+This field specifies a list of rules which determine how lines should be indented when the user presses <kbd>Enter</kbd>. When the text in a line before the cursor is matched by the regex specified in a rule's `beforeText`, that rule's `action` is applied on <kbd>Enter</kbd>.
+
+### Focus blocks
+
+```json
+{
+    "beforeText" : "^\\s*(·|\\.)\\s.*$",
+    "action" : { "indent" : "indent" }
+}
+```
+
+This rule ensures that hitting enter after starting a focus block during a tactic sequence produces a line within that focus block. I.e.,
+
+```
+  constructor
+  · intro x| <-- hit enter here
+    | <-- cursor ends up here
+```
+
 ## [`brackets`](https://code.visualstudio.com/api/language-extensions/language-configuration-guide#brackets-definition), [`autoClosingPairs`](https://code.visualstudio.com/api/language-extensions/language-configuration-guide#autoclosing), and [`surroundingPairs`](https://code.visualstudio.com/api/language-extensions/language-configuration-guide#autosurrounding)
 
 All of these fields handle brackets in different ways.

vscode-lean4/language-configuration.json

@@ -149,5 +149,11 @@
         ["*", "*"],
         ["_", "_"]
     ],
-    "wordPattern" : "([^`~@$%^&*()\\-=+\\[{\\]}⟨⟩⦃⦄⟦⟧⟮⟯‹›<>\\\\|;:\",./\\s]+)"
+    "wordPattern" : "([^`~@$%^&*()\\-=+\\[{\\]}⟨⟩⦃⦄⟦⟧⟮⟯‹›<>\\\\|;:\",./\\s]+)",
+    "onEnterRules" : [
+        {
+            "beforeText" : "^\\s*(·|\\.)\\s.*$",
+            "action" : { "indent" : "indent" }
+        }
+    ]
 }