feat(layout)!: Use *= instead of =*

[kdheepak]

This is a follow up to https://github.com/ratatui-org/ratatui-macros/pull/34

*= is more consistent with the other symbols, e.g. <=, >=.

It is also more in line with what other programming languages do.

$ python
Python 3.10.13 | packaged by conda-forge | (main, Oct 26 2023, 18:09:17) [Clang 16.0.6 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> x = 1
>>> x *= 2
>>> x
2

=* throws an error.

>>> x =* 2
  File "<stdin>", line 1
SyntaxError: can't use starred expression here
>>>

[kdheepak]

@joshka and @EdJoPaTo, I know you are not a fan of the macro but tagging you just to keep you in the loop. I'll merge this and make a new release.