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/docs/parse.md

@@ -718,7 +718,7 @@ if (auto as = context.ConsumeIf(Lex::TokenKind::As)) {
 } else {
   if (!state.has_error) {
     CARBON_DIAGNOSTIC(ImplExpectedAs, Error,
-                      "Expected `as` in `impl` declaration.");
+                      "expected `as` in `impl` declaration");
     context.emitter().Emit(*context.position(), ImplExpectedAs);
   }
   context.ReturnErrorOnState();