There are Type Errors in theia.d.ts and we don't notice it

[tsmaeder]

Bug Description:

I tried to make a theia plugin for #13085, and I noticed I was geting compile errors:

node_modules/@theia/plugin/src/theia-extra.d.ts:22:1 - error TS2668: 'export' modifier cannot be applied to ambient modules and module augmentations since they are always visible.

22 export module '@theia/plugin' {
   ~~~~~~

node_modules/@theia/plugin/src/theia.d.ts:49:1 - error TS2668: 'export' modifier cannot be applied to ambient modules and module augmentations since they are always visible.

49 export module '@theia/plugin' {
   ~~~~~~

node_modules/@theia/plugin/src/theia.d.ts:3738:9 - error TS2300: Duplicate identifier 'Production'.

3738         Production = 1,
             ~~~~~~~~~~

We never notice this in the compilation of the plugin package because *.d.ts files are not compiled and we have "skipLibCheck": true set in our compiler config.
The errors fall into two categories:

1. We use export module where in VS Code they use declare module. That makes sense, since we don't actually define any types in the theia.d.ts files, we are just describing the types to the compiler
2. We have duplicated types in some files

I'm not sure how we can detect these problems in the build process. Maybe we need to compile a sample plugin in our build process.

Steps to Reproduce:

Build anything against the package @theia/plugin with skipLibCheck: false.

Additional Information

• Operating System:
• Theia Version: