Add API Analyzer Assembly (#906)

* Start on API analyzers

* Finish GuildAccessAnalyzer

* Update build script (will this do?)

* Correct slashes

* Extrapolate DerivesFromModuleBase() to an extension method

* Quick refactoring

* Add doc file

src/Discord.Net.Analyzers/docs/DNET0001.md

@@ -0,0 +1,30 @@
+# DNET0001
+
+<table>
+<tr>
+  <td>TypeName</td>
+  <td>GuildAccessAnalyzer</td>
+</tr>
+<tr>
+  <td>CheckId</td>
+  <td>DNET0001</td>
+</tr>
+<tr>
+  <td>Category</td>
+  <td>API Usage</td>
+</tr>
+</table>
+
+## Cause
+
+A method identified as a command is accessing `Context.Guild` without the requisite precondition.
+
+## Rule description
+
+The value of `Context.Guild` is `null` if a command is invoked in a DM channel. Attempting to access
+guild properties in such a case will result in a `NullReferenceException` at runtime.
+This exception is entirely avoidable by using the library's provided preconditions.
+
+## How to fix violations
+
+Add the precondition `[RequireContext(ContextType.Guild)]` to the command or module class.