This commit switches the default parser to Prism. There are a
couple of additional changes related to this that are a part of
this as well to make this happen.

* Switch the default parser in parse.h
* Remove the Prism-specific workflow and add a parse.y-specific
  workflow to CI so that it continues to be tested
* Update a few test exclusions since Prism has the correct
  behavior but parse.y doesn't per
  https://bugs.ruby-lang.org/issues/20504.
* Skips a couple of tests on RBS which are failing because they
  are using RubyVM::AbstractSyntaxTree.of.

Fixes [Feature #20564]

[Bug #20598]

Just like [Bug #20595], Encoding#name_list and Encoding#aliases can have
their strings corrupted when Encoding.default_internal is set to nil.

Co-authored-by: Matthew Valentine-House <matt@eightbitraptor.com>

[Bug #20595]

enc_set_default_encoding will free the C string if the encoding is nil,
but the C string can be used by the encoding name string. This will cause
the encoding name string to be corrupted.

Consider the following code:

    Encoding.default_internal = Encoding::ASCII_8BIT
    names = Encoding.default_internal.names
    p names
    Encoding.default_internal = nil
    p names

It outputs:

    ["ASCII-8BIT", "BINARY", "internal"]
    ["ASCII-8BIT", "BINARY", "\x00\x00\x00\x00\x00\x00\x00\x00"]

Co-authored-by: Matthew Valentine-House <matt@eightbitraptor.com>

[Feature #18576]

Since outright renaming `ASCII-8BIT` is deemed to backward incompatible,
the next best thing would be to only change its `#inspect`, particularly
in exception messages.

* And simplify callers of get_actual_encoding().
* See [Feature #18949].
* See https://github.com/ruby/ruby/pull/6322#issuecomment-1242758474