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/diagnostics/diagnostic_consumer.cpp

@@ -21,10 +21,10 @@ auto StreamDiagnosticConsumer::HandleDiagnostic(Diagnostic diagnostic) -> void {
     *stream_ << ": ";
     switch (message.level) {
       case DiagnosticLevel::Error:
-        *stream_ << "ERROR: ";
+        *stream_ << "error: ";
         break;
       case DiagnosticLevel::Warning:
-        *stream_ << "WARNING: ";
+        *stream_ << "warning: ";
         break;
       case DiagnosticLevel::Note:
         break;