add core view extension with JTE and Thymeleaf support

flash-extensions/flash-ext-view-jte/docs/model-and-globals.md

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+# Model And Globals
+
+`ViewModel` is the per-request data container.
+
+## POJO-First Pattern
+
+Keep page state in a POJO and pass it as a single key:
+
+```java
+return ViewModel.empty()
+    .with("page", new HomePage("Flash + jte", "elorc"))
+    .with("build", "dev");
+```
+
+Then type it in template:
+
+```jte
+@import com.example.HomePage
+@param HomePage page
+@param String build
+```
+
+## Globals
+
+Register globals in extension setup:
+
+```java
+new JteExtension().addGlobal("appName", req -> "Flash")
+```
+
+Globals are available under `global` namespace in templates.
+
+`global` is reserved and cannot be used as local model key.