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/check/name_component.cpp

@@ -58,7 +58,7 @@ auto PopNameComponentWithoutParams(Context& context, Lex::TokenKind introducer)
   NameComponent name = PopNameComponent(context);
   if (name.implicit_params_id.is_valid() || name.params_id.is_valid()) {
     CARBON_DIAGNOSTIC(UnexpectedDeclNameParams, Error,
-                      "`{0}` declaration cannot have parameters.",
+                      "`{0}` declaration cannot have parameters",
                       Lex::TokenKind);
     // Point to the lexically first parameter list in the diagnostic.
     context.emitter().Emit(name.implicit_params_id.is_valid()