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/import.cpp

@@ -534,7 +534,7 @@ auto ImportNameFromOtherPackage(
   // Annotate diagnostics as occurring during this name lookup.
   DiagnosticAnnotationScope annotate_diagnostics(
       &context.emitter(), [&](auto& builder) {
-        CARBON_DIAGNOSTIC(InNameLookup, Note, "In name lookup for `{0}`.",
+        CARBON_DIAGNOSTIC(InNameLookup, Note, "in name lookup for `{0}`",
                           SemIR::NameId);
         builder.Note(loc, InNameLookup, name_id);
       });