[ReactFlightWebpackPlugin] Add support for .mjs file extension

[jennyscript]

Summary

Our builds generate files with a .mjs file extension. These are currently filtered out by ReactFlightWebpackPlugin so I am updating it to support this file extension.

How did you test this change?

I built the plugin with this change and used yalc to test it in my project. I confirmed the expected files now show up in react-client-manifest.json

[react-sizebot]

Comparing: 0c28a09...687623a

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	+0.16%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	527.72 kB	527.72 kB	=	93.07 kB	93.07 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.11%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	633.34 kB	633.34 kB	=	111.25 kB	111.25 kB
facebook-www/ReactDOM-prod.classic.js	=	671.13 kB	671.13 kB	=	117.70 kB	117.70 kB
facebook-www/ReactDOM-prod.modern.js	=	661.41 kB	661.41 kB	=	116.14 kB	116.14 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-experimental/react-server-dom-webpack/cjs/react-server-dom-webpack-plugin.js	+1.05%	14.31 kB	14.46 kB	+0.62%	3.85 kB	3.88 kB
oss-stable-semver/react-server-dom-webpack/cjs/react-server-dom-webpack-plugin.js	+1.05%	14.31 kB	14.46 kB	+0.62%	3.85 kB	3.88 kB
oss-stable/react-server-dom-webpack/cjs/react-server-dom-webpack-plugin.js	+1.05%	14.31 kB	14.46 kB	+0.62%	3.85 kB	3.88 kB
oss-experimental/eslint-plugin-react-hooks/cjs/eslint-plugin-react-hooks.development.js	=	2,006.69 kB	1,971.45 kB	=	301.58 kB	298.84 kB
oss-stable-semver/eslint-plugin-react-hooks/cjs/eslint-plugin-react-hooks.development.js	=	2,006.51 kB	1,971.27 kB	=	301.55 kB	298.81 kB
oss-stable/eslint-plugin-react-hooks/cjs/eslint-plugin-react-hooks.development.js	=	2,006.51 kB	1,971.27 kB	=	301.55 kB	298.81 kB
oss-experimental/eslint-plugin-react-hooks/cjs/eslint-plugin-react-hooks.production.js	=	2,002.10 kB	1,966.87 kB	=	300.61 kB	297.94 kB
oss-stable-semver/eslint-plugin-react-hooks/cjs/eslint-plugin-react-hooks.production.js	=	2,001.92 kB	1,966.69 kB	=	300.58 kB	297.91 kB
oss-stable/eslint-plugin-react-hooks/cjs/eslint-plugin-react-hooks.production.js	=	2,001.92 kB	1,966.69 kB	=	300.58 kB	297.91 kB

Generated by 🚫 dangerJS against 687623a