Fix buildozer :expr changing E to e (#1499)

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## Description
Fix buildozer :expr changing E to e.

When an :expr is added with a dot and an uppercase E (like
SYSTEMS.SYS1), it is mistakenly treated as a float in scientific
notation. Because it was being saved by cmdAdd as a LiteralExpr node
(which is strictly meant for numbers), the float formatter is triggered
and the E is incorrectly changed to a lowercase e (SYSTeMS.SYS1).
Changing cmdAdd to use an Ident node instead fixes this, because the
float formatter is skipped entirely. This matches how cmdSet already
handles it.