parser: fix tuple field parsing in offset_of expansion

[Pewpenguin]

Fixes #23178.

Fix tuple field parsing in offset_of expansions.

Added a parser test for tuple field paths.

[ChayimFriedman2]

The flat things makes this already-hacky thing super duper hacky. I don't want that.

Instead we should go back to basics. A float split event should just take the current pending float, and split it, so that the next two/three tokens are INT_NUMBER, DOT and perhaps another INT_NUMBER. This is not possible to do literally, but possible to emulate quite well. The plan is:

• The parser has a split_float() method. This methods does two things: emit a FloatSplitHack { ends_in_dot } event, and also changes a field in Parser to signal "we've split a float". This field will be an enum FloatSplitStage { None, BeforeFirstInt, BeforeDot, BeforeSecondInt } and peek/bump methods will consider it.
• The parser, when needed, will just call this method then proceeds to do parsing normally.
• Consumers of the parser's events will also have a field similar to the parser's to track in-flight splitting (we can't split more than one float at a time).

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[ChayimFriedman2]

Review for the parser parts. Will review the builder later.

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