This means that JSDoc types can include the full range of Typescript
types now. It also means that Typescript annotations can include JSDoc
types. This is disallowed with a new error, however. But Typescript can
still give the correct types to JSDoc that shows up in .ts files by
mistake. This can easily happen, for example with types like
```ts
var x: number? = null;
var y: ?string = null;
var z: function(string,string): string = (s,t) => s + t;
// less likely to show up, but still understood.
var ka: ? = 1;
```
In the future, I will add a quick fix to convert these into the correct
types.
Fixes#16550
It's very ambiguous in expression position, so impossible to give a
better message from the parser. For example:
let f = (x: number) => number => x + 1;
~~
Should be ':'
But the parser doesn't know that 'number' isn't an expression now.
* Created a branded type for escaped strings
Then flowed it throughout the compiler, finding and fixing a handful of
bugs relating to underscore-prefixed identifiers in the process.
Includes a test for two cases noticed - diagnostics from conflicting
symbols from export *'s, and enum with underscore prefixed member emit.
* Correctly double underscores WRT mapped types
* Add fourslash tests for other fixed issues
* use function call over cast
* Update forEachEntry type accuracy
* Just use escaped names for ActiveLabel
* Remove casts from getPropertyNameForPropertyNameNode
* This pattern has occurred a few times, could use a helper function.
* Remove duplicated helper
* Remove unneeded check, use helper
* Identifiers list is no longer escaped strings
* Extract repeated string-getting code into helper
* Rename type and associated functions
* Make getName() return UnderscoreEscapedString, add getUnescapedName()
* Add list of internal symbol names to escaped string type to cut back on casting
* Remove outdated comments
* Reassign interned values to nodes, just in case
* Swap to string enum
* Add deprecated aliases to escapeIdentifier and unescapeIdentifier
* Add temp var
* Remove unsafe casts
* Rename escaped string type as per @sandersn's suggestion, fix string enum usages
* Reorganize double underscore tests
* Remove jfreeman from TODO
* Remove unneeded parenthesis
* Support completions for JSDoc @param tag names
* Undo change to finishNode
* Don't include trailing whitespace in @param range; instead, specialize getJsDocTagAtPosition
* Allow export default interface. Related to issue 3792.
* Update exportDefaultAbstractClass test
* Allow default export to be a non-value for lookup purposes
* Add local usage to exportDefaultInterface test
