Exclude Directory.Build.rsp from VisualStudio.gitignore template

[MattKotsenas]

Reasons for making this change:

Directory.Build.rsp is a documented file that allows setting default arguments to command line builds. However, the .gitignore template ignores all *.rsp files. which causes confusion:

1. Devs write an .rsp file and if they aren't being attentive forget to commit it
2. Adding it to git requires git add --force, which some devs mistake for a destructive or not-recommended action

Thus, explicitly allow the Directory.Build.rsp file.

Links to documentation supporting these rule changes:

• https://learn.microsoft.com/en-us/visualstudio/msbuild/msbuild-response-files?view=vs-2022#directorybuildrsp