Add support for nested templates

Add support for expressions inside expressions.

Add support for templating where the output of one expression results in
another template part with expressions.

E.g., this did not work before, but compilation will now evaluate both
with expression with `$condition` and parameter substitution with
`$text`:

```
{{ with $condition }}
  echo '{{ $text }}'
{{ end }}
```

Add also more sanity checks (validation logic) when compiling
expressions to reveal problems quickly.

docs/templating.md

@@ -10,10 +10,19 @@
 
 - Expressions start and end with mustaches (double brackets, `{{` and `}}`).
   - E.g. `Hello {{ $name }} !`
-- Syntax is close to [Go Templates ❤️](https://pkg.go.dev/text/template) that has inspired this templating language.
+- Syntax is close to [Go Templates ❤️](https://pkg.go.dev/text/template) but not the same.
 - Functions enables usage of expressions.
   - In script definition parts of a function, see [`Function`](./collection-files.md#Function).
   - When doing a call as argument values, see [`FunctionCall`](./collection-files.md#Function).
+- Expressions inside expressions (nested templates) are supported.
+  - An expression can output another expression that will also be compiled.
+  - E.g. following would compile first [with expression](#with), and then [parameter substitution](#parameter-substitution) in its output.
+
+    ```go
+      {{ with $condition }}
+        echo {{ $text }}
+      {{ end }}
+    ```
 
 ### Parameter substitution
 
@@ -74,6 +83,14 @@ It supports multiline text inside the block. You can have something like:
   {{ end }}
 ```
 
+You can also use other expressions inside its block, such as [parameter substitution](#parameter-substitution):
+
+```go
+  {{ with $condition }}
+    This is a different parameter: {{ $text }}
+  {{ end }}
+```
+
 💡 Declare parameters used for `with` condition as optional. Set `optional: true` for the argument if you use it like `{{ with $argument }} .. {{ end }}`.
 
 Example: