Option to make individual aligned equations flush left like flalign

**Is your feature request related to a problem? Please describe.**
I would like to align equations, but not have them centered, but "flushed" left like the `flalign` environment.
I doesn't seem like the provided `aligned` environment supports this.

**Describe the solution you'd like:**
Maybe provide a `flaligned` environment?

**Link to or name of a (La)TeX package that provides the same feature:**
`flalign` from `amsmath`

**Describe alternatives you've considered:**
I haven't found any alternatives that can do this.

**Additional context:**

No files or tests are named. Start by locating the existing aligned environment implementation and its tests, then compare its behavior with the amsmath flalign reference. Done means providing a documented way to render individually aligned equations flush left rather than centered, with coverage for the new behavior.

Written by the indexing model from the issue text.