Can't use nullable strings in template literal

[mattjohnsonpint]

Bug description

Template literal strings can interpolate most scalar value types, including strings, numbers, booleans, etc.

However, trying to use a nullable string in a template literal currently leads to a compiler error.

Steps to reproduce

const x: string | null = "abc";  
const s = `test: '${x}'`

ERROR TS2685: The 'this' types of each signature are incompatible.
     :
   2 │ const s = `test: '${x}'`
     │                     ~
     └─ in assembly/index.ts(3,23)
     :
 644 │ toString(): String {
     │ ~~~~~~~~~~~~~~~~~~
     └─ in ~lib/string.ts(644,3)

AssemblyScript version

v0.27.36