Update basic diagnostic capitalization/punctuation (#4328)

This is a primarily automated change:

- Search & replace for capitalization
-
`(CARBON_DIAGNOSTIC\((?:\n\s+)?\w+,(?:\n\s+)?\s\w+,(?:\n\s+)?\s")([A-Z])`
    - `$1\L$2`
- Search & replace for period
-
`(CARBON_DIAGNOSTIC\((?:\n\s+)?\w+,(?:\n\s+)?\s\w+,(?:\n\s+)?\s"(?:[^)]|\n)+)\.("[,)])`
    - `$1$2`
- Limited search & replace for `ERROR: ` -> `error: ` in streamed things
- Leaving a TODO for command_line because there's more cleanup that can
be done there
- Modify diagnostic_consumer.cpp
    - ERROR -> error
    - WARNING -> warning

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parse/handle_decl_scope_loop.cpp

@@ -25,7 +25,7 @@ static auto FinishAndSkipInvalidDecl(Context& context, int32_t subtree_start)
 static auto HandleUnrecognizedDecl(Context& context, int32_t subtree_start)
     -> void {
   CARBON_DIAGNOSTIC(UnrecognizedDecl, Error,
-                    "Unrecognized declaration introducer.");
+                    "unrecognized declaration introducer");
   context.emitter().Emit(*context.position(), UnrecognizedDecl);
   FinishAndSkipInvalidDecl(context, subtree_start);
 }