Allow extensions to augment an existing language's configuration, such as surroundingPairs

[mjbvz]

From #233981

I'd like for the built-in markdown math extension to add a new set of surrounding pairs for the markdown language. Currently this doesn't seem possible as each configuration item overwrites the previous:

vscode/src/vs/editor/common/languages/languageConfigurationRegistry.ts

Line 254 in 23457f8

comments: entry.comments || result.comments,

[RedCMD]

or fix #133397
fix "autoClosingPairs" and "surroundingPairs" inside embedded languages