TEP-0099: Parameters in Script

This PR adds the problem statement for the TEP and identifies possible solutions.
The proposal will be added in a subsequent PR after discussions of alternatives.

Using `Parameter` variables directly in `script` blocks in `Tasks` is a footgun
in two ways:
- **Security**: It is easy for a `Task` _author_ to accidentally introduce a vector
  for code injection and, by contrast, difficult for a `Task` _user_ to verify that
  such an injection can't or hasn't taken place.
- **Reliability**: It is easy for a `Task` _user_ to accidentally pass in a `Parameter`
  with a character that would make the `Script` invalid and fail the `Task`, making
  the `Task` extremely fragile.

To solve the above problems, this TEP aims to:
- Introduce a safe and reliable way to access `Parameter` variables from `Scripts`,
  and update the documentation and *Tekton Catalog* with the new approach.
- Disallow use of `Parameter` variables directly in `script` blocks of `Steps` in
  *Tekton Pipelines V1 API*.

References:
* Issues:
  * tektoncd/pipeline#3226
  * tektoncd/triggers#675
  * tektoncd/plumbing#971
* [Catalog Guidance to Avoid Using `Parameters` in `Script` Blocks](https://github.com/tektoncd/catalog/blob/main/recommendations.md#dont-use-interpolation-in-scripts-or-string-arguments)
* Tekton Enhancement Proposals:
  * [TEP-0017: Shell-Escaped Parameters](#208)
  * [TEP-0023: Implicit Parameters](https://github.com/tektoncd/community/blob/main/teps/0023-implicit-mapping.md)

Co-authored-by: Scott Seaward <[email protected]>