Don't declare private fields in user facing definition files

When tsdoc is used from multiple libraries each having a slightly different version, private fields cause type incompatibilites. For example:

src/api-documenter/markdown/CustomMarkdownEmitter.ts:167:7 - error TS2345: 
Argument of type 
'import("node_modules/@microsoft/tsdoc/lib/nodes/DocDeclarationReference").DocDeclarationReference' 

is not assignable to parameter of type 
'import("node_modules/@microsoft/api-extractor-model/node_modules/@microsoft/tsdoc/lib/nodes/DocDeclarationReference").DocDeclarationReference'.

Types have separate declarations of a private property '_tagDefinitions'.

This is due to slightly different versions requested by the libraries, as shown in yarn.lock:

"@microsoft/tsdoc@0.12.14":
  version "0.12.14"
  resolved "https://registry.yarnpkg.com/@microsoft/tsdoc/-/tsdoc-0.12.14.tgz#0e0810a0a174e50e22dfe8edb30599840712f22d"
  integrity sha512-518yewjSga1jLdiLrcmpMFlaba5P+50b0TWNFUpC+SL9Yzf0kMi57qw+bMl+rQ08cGqH1vLx4eg9YFUbZXgZ0Q==

"@microsoft/tsdoc@^0.12.14":
  version "0.12.18"
  resolved "https://registry.yarnpkg.com/@microsoft/tsdoc/-/tsdoc-0.12.18.tgz#7e7aadc7009e57a25fbf6c9f48b44c2fe30ada88"
  integrity sha512-3rypGnknRaPGlU4HFx2MorC4zmhoGJx773cVbDfcUgc6zI/PFfFaiWmeRR6JiVyKRrLnU/ZH0pc/6jePzy/QyQ==

Yarn always prefers the newest version possible for each module - however, private fields make even "revision" versions incompatible.

Start with the TypeScript definitions for DocDeclarationReference and the CustomMarkdownEmitter.ts error location, then inspect the conflicting @microsoft/tsdoc entries in yarn.lock. Determine how the user-facing definitions should avoid incompatibility between slightly different TSDoc versions. Done means the reported TS2345 incompatibility no longer occurs when the libraries resolve different revision versions.