Parser: clean up handling of variable patterns.

The listener API for variable patterns is split into three separate
functions, to handle the three separate behaviors:

- `handleAssignedVariablePattern` for variable names appearing in an
  assignment context (these assign to an existing variable upon a
  successful match).

- `handleDeclaredVariablePattern` for variable declarations appearing
  in a declaration or matching context (these cause a new variable
  name to come into scope).

- `handleWildcardPattern` for wildcards in any context (these don't
  capture the matched value).

Also, responsibility is shifted to the parser for reporting the
following error conditions:

- VariablePatternKeywordInDeclarationContext (e.g.
  `var (var x) = ...;`)

- PatternAssignmentDeclaresVariable (e.g. `[x, var y] = ...;`)

Previously these errors were detected by the implementations, and
weren't fully covering all possible error scenarios.

In the case of VariablePatternKeywordInDeclarationContext, the
listener method `handleDeclaredVariablePattern` is called instead of
`handleAssignedVariablePattern`.  This ensures that no tokens are
dropped from the analyzer AST.  The CFE uses the `inAssignmentPattern`
argument of `handleDeclaredVariablePattern` to distinguish this error
recovery case from a legitimate declared variable pattern.

Fixes #51868.

Bug: https://github.com/dart-lang/sdk/issues/51868
Change-Id: I28ec679b73d64033166721c6460be35f15e23171
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/291583
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

https://dart.googlesource.com/sdk/+/104ac30cf4890db59342ff0c7b73d1dec68a02b3