remove the regular dependency on package:matcher

[devoncarew]

• remove the regular dependency on package:matcher
• https://github.com/dart-lang/code_builder/issues/405

Creating as a draft PR - I'm trying to determine what would be impacted by this change.

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[devoncarew]

Based on package usage on pub.dev and a presubmit in google3, I think a reasonable path forward would be to:

• deprecate the equalsDart function and publish a new package version
• remove equalsDart and EqualsDart (this PR) and publish a new major version
• recommend that clients who do want a matcher similar to the current equalsDart one in-line something like https://github.com/dart-lang/code_builder/pull/457/files#diff-a069bad576c5b11a048b0992ff9e46d3861f179d3b2a5c5be71215de051210a6 into their projects (i.e., we no longer publish that matcher)