Text literals: Accept unpaired-surrogate escape codes.

[kazcw]

Pull Request Description

Unpaired surrogates are not allowed by Unicode, but they occur in practice because many systems accept them; for example, they may be present in filenames on Windows (which are otherwise constrained to UTF-16).

Programs written in Enso should be able to work with them, if only because they represent edge cases that should be tested when converting encodings and at other system boundaries.

Before this change, escapes codes for the range of surrogate codepoints were treated as uninterpretable escapes; they now are interpreted as their specified codepoint values.

(Fixes an issue found while writings tests for #9456)

Important Notes

• Generalize the representation of interpreted-text-escapes in the lexer, so that we are not tied to the strict Unicode of Rust's str.
• Move some doc-comment code from the parser to test utilities.
• Simplify token serialization.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.

[JaroslavTulach]

Using Escape 13 instead of Escape '\n' seems fine to me.