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_base.cpp

@@ -17,7 +17,7 @@ auto HandleBaseAfterIntroducer(Context& context) -> void {
     // based on whether we're in a class, whether we have an `extend`
     // modifier, and the following tokens.
     CARBON_DIAGNOSTIC(ExpectedAfterBase, Error,
-                      "`class` or `:` expected after `base`.");
+                      "`class` or `:` expected after `base`");
     context.emitter().Emit(*context.position(), ExpectedAfterBase);
     context.RecoverFromDeclError(state, NodeKind::BaseDecl,
                                  /*skip_past_likely_end=*/true);