feat(@schematics/angular): add solutions style tsconfig structure

[alan-agius4]

In version 3.9, TypeScript introduced the concept of "Solutions Style" tsconfig to improve developer experience.

More info: https://devblogs.microsoft.com/typescript/announcing-typescript-3-9-rc/#solution-style-tsconfig

Closes#17493 and closes#8138

[alan-agius4]

Blocking until support for TS 3.9 is landed in the framework. None the less, we should get this in a ready to be merged when that happens.

[kyliau]

We had a discussion whether the filename convention should be tsconfig-base.json or tsconfig.base.json. The former follows the same pattern used in the TS repo, whereas the latter allows the Angular workspace to enforce a consistent file naming.
Another concern is the convention used in google3. However, since google3 does not have plan to introduce solution-style tsconfig, the consideration is moot here.
We've decided to with tsconfig.base.json to ensure a consistent naming across Angular workspace.
cc: @IgorMinar

[IgorMinar]

lgtm!

[IgorMinar]

We've decided to with tsconfig.base.json to ensure a consistent naming across Angular workspace.

Sounds great! thanks for documenting the decision making process.

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