Strict mergeInto

[Stuonts]

We have a large project with a lot of .js files and both dynamic linking and static linking config.

There are 2 issues with mergeInto:

1. There is possibility of functions with the same names in different .js files can overwrite each other unintentionally
2. There is an issue with missing .sig when dynamic linking config is used, while it works with static linking, the most of work is done with static linking config, so it's possible to forget __sig addition and have errors after switching to dynamic linking config

It would be nice to have strict mergeInto (it could be mergeIntoStrict) that will generate errors in case of

• function names duplicates
• error in case of missing __sig
  it's better to have error than warning in mergeIntoStrict (but would be nice to have warning in mergeInto) because warning can be easily missed

related issue and PR:
#16359
#16364