Chaining of prefix and postfix operators

[jacobpake]

Parsers created with buildExprParser will try to chain prefix and postfix operators.

Previously, it was not possible to chain prefix operators, e.g. not not True, though this is was the behaviour for infix operators.

Consumers of this library may rely on the old behaviour.

Currently, this PR is a breaking change.

It should be considered if chaining/non-chaining is configurable on a per-operator basis (similar to how associativity is configurable for infix operators).

Another option is to add new constructor(s) to Operator, e.g. PrefixChained, PostfixChained to avoid the breaking change.

Resolves #241

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)