feat: type import API for TypeScriptWriter

[kuhe]

• adds new method signatures to TypeScriptWriter: addTypeImport, addRelativeTypeImport, addTypeImportSubmodule corresponding to their regular addImport counterparts. This allows the code generation author to make use of the TypeScript feature type-only-imports
• reorganizes how imports and type imports are emitted to the TypeScriptWriter::toString method, matching the simple-import-sort eslint plugin in use by this repository's and AWS SDK's TypeScript source code.
  • i.e., alphabetical ignoring case, separated sections for package and relative imports

See https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-8.html#type-only-imports-and-export