Allow plugins to run before or after expressions plugin

[wzrdjstr]

Adds a new {before:[], after: []} option to the plugins config, which allows plugins to run before or after posthtml-expressions.

This is helpful when a plugin needs to modify the AST before expressions are evaluated.

For example:

Maizzle uses posthtml-components under the hood.

I have a posthtml plugin that converts Maizzle templates into my web app's native template format (Twig). During conversion, it replaces <if> nodes marked with a twigify attribute with the corresponding {%- if -%} Twig syntax. This plugin must run before posthtml-expressions, because I don't want the tagged <if> nodes to be evaluated and because <if> nodes are removed from the AST when expressions are evaluated, which prevents plugins from processing them later.

I also have another plugin that removes <script locals> before they're evaluated. This allows me to use <script locals> to inject static placeholder data into prototype templates, while preserving uninterpolated {{ }} delimiters in the converted template so they can be filled by my app. This plugin needs to run before posthtml-expressions, so <script locals> are removed before they're evaluated.

This change is backwards-compatible; docs and tests also updated.

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Please note the plugin run order was changed after the 2022 refactor (45f00f5). Prior to the refactor, plugins ran before posthtml-expressions. This can be seen in code that's currently commented-out:

posthtml-components/src/index.js

Line 219 in e1a2361

// const plugins = [...options.plugins, expressions(options.expressions)];

I'm guessing this was an optimization, e.g. to skip processing nodes that are ultimately removed from the AST by an expression, but just wanted to give you a heads up in case there was some other reason.

[cossssmin]

Hey, this looks great, thanks for working on it!

[cossssmin]

https://github.com/posthtml/posthtml-components/releases/tag/v2.3.0