wp-signup page lacks a role="main"

[SteelWagstaff]

The wp-signup.php page lacks a role="main" landmark. According to w3c guidelines, each webpage should have one main landmark, designated by the role="main" attribute. See ​https://www.w3.org/TR/wai-aria-practices/examples/landmarks/main.html. It seems to me that the existing container div <div class="mu_register wp-signup-container"> https://core.trac.wordpress.org/browser/trunk/src/wp-signup.php#L89 would be a good candidate to receive the role="main" designation in this case. Patch attached (this is my first time contributing, so happy to take instruction if any part of this could be improved).

[SteelWagstaff]

Adds role="main" to existing container div on the wp-signup.php page