Print more macro information in DefMap dumps

[Zalathar]

While working towards #21043, I found myself wanting to be able to see more information about macros in snapshot tests that use DefMap dumps.

This PR therefore makes a few changes to the dump output:

• Make the dump output nicer to read (at least to my eyes) by using bulleted lists, making sure names are surrounded by space on both sides, using full words to indicate namespace and import style, and adding parens around (block scope).
• When dumping macros, also indicate which macro sub-namespace they belong to (function-like ! or attribute/derive #).
• When dumping scopes, also dump any legacy-textual-scope macros visible at the end of the scope, marked with a (legacy) prefix.

[Veykril]

Thanks!