[babel 8] Cleanup parser plugins

[nicolo-ribaudo]

• Remove from https://github.com/babel/babel/blob/main/packages/babel-parser/src/typings.d.ts all those that are enabled by default
  • Note that many of then are not explicitly marked as "deprecated" or "this is enabled by default"
• importAssertions should be removed, as well as the deprecatedAssertKeyword option in importAttributes
• importReflection is now sourcePhaseImports
• decimal doesn't have syntax anymore
• The smart and minimal pipeline proposals are gone for sure
• Flow enums option should be enabled by default
• record&tuple is not going to use {| ... |} (Remove syntaxType option for record-and-tuple (parser&plugin) #16458)

[JLHwung]

If record and tuple will use #, then we should remove the support of hack pipeline with topic token # right? These two are conflicted with each other.

[nicolo-ribaudo]

The current status is that:

• the future of R&T is uncertain, but if it happens and if it has syntax, the only plan is to use #{
• the future of pipeline is uncertain, but if it happens all possible tokens are still on the table

[JLHwung]

Well in that case we still have to support both features. Thus the error message here

babel/packages/babel-parser/src/plugin-utils.ts

Line 164 in 71c247a

'Plugin conflict between `["pipelineOperator", { proposal: "hack", topicToken: "#" }]` and `["recordAndtuple", { syntaxType: "hash"}]`.',

should be refined since syntaxType won't be an option in Babel 8.