[FR]: --loader option

### What is the current behavior?

Not sure how to use `--loader` option, e.g. `--loader:.svg=text`

### Describe the feature

Allow users to set `--loader:.svg=text`

### Fund our work

- [ ] Sponsor our open source work by donating a [feature bounty](https://opencollective.com/aspect-build/)

Start by locating how rules_esbuild assembles esbuild arguments and how existing loader settings are exposed; the issue gives no file or test entry point. Reproduce the requested --loader:.svg=text case, then add coverage showing that the configured option reaches esbuild and works as intended.

Written by the indexing model from the issue text.