[api-extractor] Handling excerptTokens with curly braces

[natalieethell]

Api-extractor doesn't seem to correctly handle the case in which an extends block has open and closed curly braces within it. For example, take an interface with the following declaration:

export interface A extends B<C & {d: D}> {
  ...
}

Since nodeToStopAt is defined as '{' for interfaces, the excerptTokens written to the api.json file end at '&', instead of after '>'.

In addition, the extendsTokenRanges in the api.json file we're getting for this interface look a little strange:

"extendsTokenRanges": [
  {
    "startIndex": 0,
    "endIndex": 0
  },
  {
    "startIndex": 0,
    "endIndex": 0
  }
]

[octogonz]

We should definitely fix this.

Currently the end token is specified using nodeToStopAt which is a ts.SyntaxKind. Maybe it should be an actual node.