implement a copy loader

[evanw]

You can configure the "loader" for a specific file extension in esbuild, which is a way of telling esbuild how it should treat that file. For example, the text loader means the file is imported as a string while the binary loader means the file is imported as a Uint8Array. If you want the imported file to stay a separate file, the only option was previously the file loader (which is intended to be similar to Webpack's file-loader package). This loader copies the file to the output directory and imports the path to that output file as a string. This is useful for a web application because you can refer to resources such as .png images by importing them for their URL. However, it's not helpful if you need the imported file to stay a separate file but to still behave the way it normally would when the code is run without bundling.

With this PR, there is now a new loader called copy that copies the loaded file to the output directory and then rewrites the path of the import statement or require() call to point to the copied file instead of the original file. This will automatically add a content hash to the output name by default (which can be configured with the --asset-names= setting). You can use this by specifying copy for a specific file extension, such as with --loader:.png=copy.

Fixes: #2255