[Ruby 3.0] Add support for find pattern

[vinistock]

Changes from whitequark

• - lexer.rl: don't perform lookahead after tASSOC. whitequark/parser#764
• + ruby28.y: add find pattern. whitequark/parser#714

This PR adds support for the find pattern, which is another variation of the pattern matching syntax.

The syntax works as follows: it allows finding specific matches inside structures and (optionally) assigning it to a variable. For example,

foo = [1, 2, 3, 4, 5]

case foo; in [*x, 3 => a, *y] then true; end # foo has the element 3 inside, therefore this will return true and set a = 3

case foo; in [*x, 7 => a, *y] then true; end # foo does not have the element 7, therefore this will raise NoMatchingPatternError

Motivation

Part of the effort to support Ruby 3.0.

Test plan

See included automated tests.

[jez]

We have a policy of testing changes to Sorbet against Stripe's codebase before
merging them. I've kicked off a test run for the current PR. When the build
finishes, I'll share with you whether or how it failed. Thanks!

Stripe employees can see the build results here:

→ https://go/builds/bui_JwEGm4dUTxuhPS
→ https://go/builds/bui_JwEGsy8spQriOj

[jez]

Thanks!