[IMP] compiler: add support for the .translate suffix

Previously, if you wanted to pass a prop and have it be translated, you
had to either to the translation manually in JS, or use a workaround
with t-set and a body so that Owl would translate it for you, and then
pass the t-set variable as a prop. This is quite inconvenient and is a
common use case.

This commit introduces the `.translate` suffix to solve this issue. When
a prop uses this suffix, it is treated as a string instead of a JS
expression, avoiding the need for quotes as well as their escaping and
allowing extraction tools such as babel to generate a clean string as
the term's translation id. This is also more ergonomic. This suffix is
available for both component props and slot props.

This change will still require some work in Odoo to correctly extract
the terms for props using this suffix.

doc/reference/props.md

@@ -140,6 +140,28 @@ class SomeComponent extends Component {
 The `.bind` suffix also implies `.alike`, so these props will not cause additional
 renderings.
 
+## Translatable props
+
+When you need to pass a user-facing string to a subcomponent, you likely want it
+to be translated. Unfortunately, because props are arbitrary expressions, it wouldn't
+be practical for Owl to find out which parts of the expression are strings and translate
+them, and it also makes it difficult for tooling to extract these strings to generate
+terms to translate. While you can work around this issue by doing the translation in
+JavaScript, or by using `t-set` with a body (the body of `t-set` is translated),
+and passing the variable as a prop, this is a sufficiently common use case that Owl
+provides a suffix for this purpose: `.translate`.
+
+```xml
+<t t-name="ParentComponent">
+    <Child someProp.translate="some message"/>
+</t>
+```
+
+Note that the content of this attribute is _NOT_ treated as a JavaScript expression:
+it is treated as a string, as if it was an attribute on an HTML element, and translated
+before being passed to the component. If you need to interpolate some data into the
+string, you will still have to do this in JavaScript.
+
 ## Dynamic Props
 
 The `t-props` directive can be used to specify totally dynamic props:

doc/reference/slots.md

@@ -201,16 +201,17 @@ use this `Notebook` component:
 
 ```xml
 <Notebook>
-  <t t-set-slot="page1" title="'Page 1'">
+  <t t-set-slot="page1" title.translate="Page 1">
     <div>this is in the page 1</div>
   </t>
-  <t t-set-slot="page2" title="'Page 2'" hidden="somevalue">
+  <t t-set-slot="page2" title.translate="Page 2" hidden="somevalue">
     <div>this is in the page 2</div>
   </t>
 </Notebook>
 ```
 
-Slot params works like normal props, so one can use the `.bind` suffix to
+Slot params works like normal props, so one can use suffixes like `.translate`
+when a prop is a user facing string and should be translated, or `.bind` to
 bind a function if needed.
 
 ## Slot scopes