feat(hogql): placeholder expressions (part 1)

[mariusandra]

Problem

Pulls out the first chunk from #25040

Changes

• Convert placeholders from being LBRACE nestedIdentifier RBRACE to being LBRACE columnExpr RBRACE.
• There should be zero functional changes. We will now be able to parse more complex syntax within placeholder tags, but we won't do anything with it. This is a separate PR mostly because releasing a new parser is its own dance.
• Most places that use placeholders call the helper method/property node.field, so there was little to change.

How did you test this code?

Updated a lot of tests

[posthog-bot]

It looks like the code of hogql-parser has changed since last push, but its version stayed the same at 1.0.40. 👀
Make sure to resolve this in hogql_parser/setup.py before merging!