message: update BREAKING.md for new tsc version
  Add a line entry for new TypeScript breaking changes.
diff: +# 0.13 Breaking Changes
  +
  +- [Fluid Packages Require Consumers on TypeScript `>=3.6`](##Fluid-Packages-Require-Consumers-on-TypeScript->=3.6)
  +
  +## Fluid Packages Require Consumers on TypeScript `>=3.6`
  +
  +Fluid now requires consumers of our packages to use a TypeScript compiler version `>=3.6`. The Fluid `./packages` repo has upgraded to TypeScript `3.7.4`. TypeScript 3.7 has a breaking change to the `.d.ts` format having to do with getters and setters and is part of an effort to do [Class Field Mitigations](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#class-field-mitigations).
  +
  +TypeScript now emits `get/set` accessors in `.d.ts` files. TypeScript versions `3.5` and prior do not know how to read these and throw the below error when compiling. TypeScript version `3.6` is forwards compatible but does not emit the accessors.
  +
  +```text
  +"error TS1086: An accessor cannot be declared in an ambient context."
  +```
  +
  +More about the changes:
  +
  +- [Class Field Mitigations](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#class-field-mitigations)
  +- [Full list of TypeScript changes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html)
  +
  # 0.12 Breaking Changes
  - [Packages moved from packages to server](#Packages-moved-from-packages-to-server)
  