Add support for linewidth in legend frame parsing

[K-Mirembe-Mercy]

This PR adds support for linewidth and lw as aliases for controlling the legend frame edge width.

Previously, only edgewidth was supported. This improves usability and aligns behavior with matplotlib conventions.

Changes:

• Updated _parse_frame() to accept linewidth and lw.

[cvanelteren]

Hi @K-Mirembe-Mercy. Good start but this doses not solve the issue yet. While parseframe now accepts the new keywords, we need to update the parts of the code so that it updates the frame.

You can have a look how that is performed for legend.geo. After the code is implemented (or before), we need to add unit tests that will ensure correctness of to e added code.
Hope this helps.

[cvanelteren]

The commit marked as 0 is also a bit curious. Could you restore it to the one before.