This bugfix addresses an issue where embedded Unison code in UCM was expected to be present in the active codebase when the `display` command was used render `Doc` values.

First, a few \[hidden\] definitions necessary for typechecking a simple Doc2.

``` ucm
scratch/main> add

  ⍟ I've added these definitions:
  
    type Doc2
    type Doc2.SpecialForm
    type Doc2.Term
    structural type Optional a
      (also named builtin.Optional)
    syntax.docParagraph       : [Doc2] -> Doc2
    syntax.docUntitledSection : [Doc2] -> Doc2
    syntax.docWord            : Text -> Doc2

```
Next, define and display a simple Doc:

``` unison
README = {{
Hi
}}
```

``` ucm
scratch/main> display README

  Hi

```
Previously, the error was:

``` 
⚙️   Processing stanza 5 of 7.ucm: PE [("die",SrcLoc {srcLocPackage = "unison-parser-typechecker-0.0.0-He2Hp1llokT2nN4MnUfUXz", srcLocModule = "Unison.Runtime.Interface", srcLocFile = "src/Unison/Runtime/Interface.hs", srcLocStartLine = 118, srcLocStartCol = 18, srcLocEndLine = 118, srcLocEndCol = 60})] Lit
  AnnotatedText (fromList [Segment {segment = "Unknown term reference: #4522d", annotation = Nothing}])
```

but as of this PR, it's okay.