fix: lg files auto add placeholder for missing templates

[zhixzhan]

Description

Our current lg CRUD operations are sync across multiple locales, to keep them structure aligns. but if the bot loaded initial status is not structure align (e.g migration from old version, edited in third party editor), authoring in Composer will meet a error, and so does running the bot.

For example a bot have two locale lg files:

<!-- a.en-us.lg -->                  |              <!-- a.zh-cn.lg --> 
# SendActivity_12345                 |
-hi                                  |

The searching scope of dialog used but missing templates can be .<*locale>.lg. for example

And dialog 'a' is using a lg called 'SendActivity_12345' , will do auto add to lg files missing this template ( here is zh-cn, same to other locales fr-fr etc.).

After auto repair, two locale lg files will looks like:

<!-- a.en-us.lg -->                  |              <!-- a.zh-cn.lg --> 
# SendActivity_12345                 |              # SendActivity_12345
-hi                                  |              -

Task Item

refs #5441

Screenshots

Real-time refreshing "readOnly" editor:

[coveralls]

Coverage increased (+0.03%) to 55.192% when pulling b853627 on zhixzhan/lg-multilang-files-sync into 7bafd30 on main.