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_code_block.cpp

@@ -19,7 +19,7 @@ auto HandleCodeBlock(Context& context) -> void {
                         /*has_error=*/true);
 
     // Recover by parsing a single statement.
-    CARBON_DIAGNOSTIC(ExpectedCodeBlock, Error, "Expected braced code block.");
+    CARBON_DIAGNOSTIC(ExpectedCodeBlock, Error, "expected braced code block");
     context.emitter().Emit(*context.position(), ExpectedCodeBlock);
 
     context.PushState(State::Statement);