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_choice.cpp

@@ -19,7 +19,7 @@ auto HandleChoiceDefinitionStart(Context& context) -> void {
   if (!context.PositionIs(Lex::TokenKind::OpenCurlyBrace)) {
     if (!state.has_error) {
       CARBON_DIAGNOSTIC(ExpectedChoiceDefinition, Error,
-                        "Choice definition expected.");
+                        "choice definition expected");
       context.emitter().Emit(*context.position(), ExpectedChoiceDefinition);
     }
 
@@ -54,7 +54,7 @@ auto HandleChoiceAlternative(Context& context) -> void {
                                        NodeKind::IdentifierName)) {
     if (!state.has_error) {
       CARBON_DIAGNOSTIC(ExpectedChoiceAlternativeName, Error,
-                        "Expected choice alternative name.");
+                        "expected choice alternative name");
       context.emitter().Emit(*context.position(),
                              ExpectedChoiceAlternativeName);
     }