Formalize templating syntax

[christophermaier]

Description

Spurred by #4615 and the subsequent fix in #4620, we have a bit of work to do to tighten up our templating syntax.

As detailed in #4620, the Rust handlebars crate we're currently using changed how it processes array access; it used to use {{foo[0]}}, which is Habitat's syntax, but it was changed in version 0.29.0 to use the Handlebars-standard {{foo.[0]}} dot notation instead. Testing at http://tryhandlebarsjs.com suggests that this is, in fact, the correct Handlebars syntax for array access (foo[0] and foo.0 both fail).

This in turn suggests that we would need to update our templating engine to accept the old format as well as the newer, correct format in Habitat templates (otherwise we'll be stuck on version 0.28.3 of the handlebars crate forever).

This further suggests that we should have a linter that can validate templates, and suggest updates to the correct syntax when appropriate (we've had a linter request before: #1779). The regression above also indicates that we need a more complete test suite of our rendering engine, ensuring that it can properly process anything we consider to be a valid template.

Acceptance Criteria

[baumanj]

Confirmed valid in v0.57

[christophermaier]

Added this as a 1.0 task, as I'd like to avoid potentially breaking syntax changes after that.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. We value your input and contribution. Please leave a comment if this issue still affects you.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. We value your input and contribution. Please leave a comment if this issue still affects you.