Feature Request: remove line and character position data from sourcemap

Source maps are very large.
I only need the original filename (and original file path) for debugging stack traces.

There is an option to remove the `sourceContent`: https://esbuild.github.io/api/#sources-content
```
{
"version": 3,
"sources": ["bar.js", "foo.js"],
"sourcesContent": ["bar()", "foo()\nimport './bar'"],
"mappings": ";AAAA;;;ACAA;",
"names": []
}
```

A good feature is to limit the mappings data to just the original filename and original filepath WITH no line number and character information (Or map everything to line 0 character 0 of a given source file)

Start by tracing esbuild's source-map generation and how the `mappings` field encodes original positions. Define the reduced mapping behavior and its interaction with `sources` and `sourcesContent`, then add coverage showing that generated maps retain original filenames and paths without line or character positions.

Written by the indexing model from the issue text.