Modernise, and incorporate improvements from localstack downstream

[purcell]

The downstream localstack fork contains a number of changes and improvements, while also breaking compatibility with velocity in favour of matching the behaviour of AWS' own fork of velocity used for templating in services like AppSync.

Additionally, the downstream fork adds golden testing against those AWS services to ensure compatibility.

The current intention of this PR is to attempt to:

• Backport all downstream improvements to compatibility with core Velocity functionality.
• Where possible, capture AWS-specific changes as config options or override mechanisms, so that downstream can use this version of Airspeed with those tweaks enabled.
• Add a golden testing mechanism here, and use it to validate Airspeed against Velocity.
• Provide enough support that downstream can use golden testing to compare Airspeed against AWS' template mechanisms. (This PR will not introduce AWS-specific golden testing in this repo.)

The next current challenge is to backport enhanced assignment (e.g. $foo[1].bar = 3) — downstream uses a concise but limited approach for this, and I believe there's a general solution lurking which would also allow direct support for the case that downstream's REPLACE_FORMAL_TEXT trick serves.