https://www.typescriptlang.org/docs/handbook/tsconfig-json.html
The presence of a tsconfig.json file in a directory indicates that the directory is the root of a TypeScript project.
文件夹里若存在 tsconfig.json, 说明这个文件夹是 TypeScript 项目的根文件夹。
It specifies the root files and the compiler options required to compile the project.
该文件指定了 root 文件，以及为了编译项目所必须的编译选项。
By invoking tsc with no input files, in which case the compiler searches for the tsconfig.json file starting in the current directory and continuing up the parent directory chain.
compilerOptions：can be omitted, in which case the compiler’s defaults are used.
