chore(common): add restructure .d.ts for esm

[mcdurdin]

Transitive deps get the following error on tsc -b, e.g. kmc-package depends on common-types which depends on restructure:

../../../node_modules/@keymanapp/common-types/src/kmx/kmx.ts:1:20 - error TS7016: Could not find a declaration file for module 'restructure'. 'C:/Projects/keyman/app/node_modules/restructure/index.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/restructure` if it exists or add a new declaration (.d.ts) file containing `declare module 'restructure';`
1 import * as r from 'restructure';
                     ~~~~~~~~~~~~~

../../../node_modules/@keymanapp/common-types/src/kmx/kmx-file-reader.ts:2:20 - error TS7016: Could not find a declaration file for module 'restructure'. 'C:/Projects/keyman/app/node_modules/restructure/index.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/restructure` if it exists or add a new declaration (.d.ts) file containing `declare module 'restructure';`
2 import * as r from 'restructure';
                     ~~~~~~~~~~~~~

Discussed at microsoft/TypeScript#50223, fix is to add the appropriate types to exports key in package.json for restructure, something like this:

"exports": {
    ".": {
      "import": {
        "types": "./src/restructure-esm.d.ts",
        "default": "./dist/index.js"
      },
    }
  },
"types": "./src/restructure.d.ts",

[mcdurdin]

See also foliojs/restructure#55