Fix nested escaped dollarsigns

[DrJosh9000]

What

Change how escaped expansions work back to only "expanding" to a $ - but still capture the potential identifier or brace expression that follows.

Given BAR=bar, $${FOO:$BAR} expands to ${FOO:bar} and $${FOO:$$BAR} expands to ${FOO:$BAR} (the pre-#10) behaviour).

While I'm here, do a bunch of work on the tests.

Why

Fixes #14

How

Originally, \$ and $$ were parsed into a single $ as plain text, and it didn't affect the interpretation of the following text.

We had the need to find potential "run-time" interpolations in buildkite-agent, so "escaped expansions" were added that parse like "normal" variable expansions, but produce different text when expanded. EscapedExpansion then had to be expanded (ahem) to handle brace expressions as well.

Here are the key changes in this PR:

 func (e EscapedExpansion) Expand(Env) (string, error) {
-       return "$" + e.Identifier, nil
+       return "$", nil
 }

and here.

Previously, EscapedExpansion would capture the identifier or brace expression that followed it. This caused problems for the interpretation of nested expansions: if the escaped dollarsign were treated simply as text as it should be, then expressions nested within the brace following the escaped dollarsign could be parsed and expanded as intended. Instead, because the brace was captured by EscapedExpansion, and expanded verbatim, that wouldn't happen.

Now, EscapedExpansion goes back to expanding only to $. Instead of capturing the following identifier or brace for expansion, it records what it was, then the parser restarts from the same position as text.

[moskyb]

this is awesome - way more robust than my initial implementation! hopefully this clears up the issues we've been seeing.

[moskyb]

this is awesome - way more robust than my initial implementation! hopefully this clears up the issues we've been seeing.

[moskyb]

the approval so nice, i made it twice