[LG] Inject LG templates as a global expression function

[cosmicshuai]

close: #3652
In this PR, users can register namespace for a lg file. Also, they can register some templates as a expression functions, which can be applied easily in dialog.
Here is an example to use these options,

!# @namespace = foo
!# @exports = template1, template2

After registration, in a dialog file, such usage "property": "=foo.template1()" is enabled, foo.template1() just behaves like a customized function.

The limitation is this kind of 'LG template function' can not read any scope from dialogStateManager, which is the same as prebuilt functions.

For example,
we have a memory in dialogStateManager:
user.tasks: [ "car", "washing", "food", "laundry" ]
if you to join the list of user.tasks, you have to define the LG file as

> !# @namespace = foo
> !# @Export s= joinList

#joinList(tasks)
- ${join(tasks, " ,")} 

Then call the function as

=foo.joinList(user.tasks)

[tomlm]

🕐

revoking review

[xieofxie]

Will this apply for the dialogs under dialog having this lg or all dialogs? Thanks

[cosmicshuai]

@xieofxie, all dialogs

[xieofxie]

@xieofxie, all dialogs

So we will load all lgs when bot is starting? And if it is for all lgs, it means we could use another lg's function without import it?

[cosmicshuai]

@xieofxie, all dialogs

So we will load all lgs when bot is starting?

Yes, all LGs are loaded when bot starting. All exported templates can be used as a prebuilt functions under all dialogs.

[tomlm]

[boydc2014]

@cosmicshuai let's make sure our options are not case-sensitive before we take this in