build: support source map in build and dev scripts

.github/contributing.md

@@ -106,9 +106,13 @@ Multiple formats can be specified as a comma-separated list:
 yarn build runtime-core -f esm,cjs
 ```
 
+#### Build with Source Maps
+
+Use the `--sourcemap` or `-s` flag to build with source maps. Note this will make the build much slower.
+
 #### Build with Type Declarations
 
-The `--types` flag will generate type declarations during the build and in addition:
+The `--types` or `-t` flag will generate type declarations during the build and in addition:
 
 - Roll the declarations into a single `.dts` file for each package;
 - Generate an API report in `<projectRoot>/temp/<packageName>.api.md`. This report contains potential warnings emitted by [api-extractor](https://api-extractor.com/).
@@ -129,6 +133,8 @@ $ yarn dev
 
 - The `dev` script supports specifying build format via the `-f` flag just like the `build` script.
 
+- The `dev` script also supports the `-s` flag for generating source maps, but it will make rebuilds slower.
+
 ### `yarn test`
 
 The `yarn test` script simply calls the `jest` binary, so all [Jest CLI Options](https://jestjs.io/docs/en/cli) can be used. Some examples: