[#395] Switch to home-grown string interpolation

Description

Not updating changelogs, want to see if it passes all tests first (particularly doctests, since running those locally is a bit of a chore)

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Problem: interpolate, which we use for string interpolation, is quite dependency-heavy, and we do not actually use the features those dependencies are required for.

Solution: make our own. See "Morley.Util.StringInterpolation" for documentation.

One feature we lose is the ability to use expressions inside interpolated strings, but we didn't actually use it anywhere, and anyway it's arguably way cleaner to write, e.g.

[md|${one} ${two}|] where
  one = pretty bigExpression
  two = pretty evenBiggerExpression

than inline those expressions in the string.

Related issue(s)

Resolves #395 (closed)

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