Support both Sprockets and Webpacker

[kylefox]

See discussion at #102 for context.

This PR introduces two new helpers inline_svg_tag and inline_svg_pack_tag which will search for SVG files using Sprockets and Webpacker, respectively.

The logic for finding and rendering an SVG has been moved into a private method, render_inline_svg.

The inline_svg helper will continue to function as expected (uses any configured asset_finder), but now raises a deprecation warning:

DEPRECATION WARNING:
  `inline_svg` is deprecated and will be removed from inline_svg 2.0
  (use `inline_svg_tag` or `inline_svg_pack_tag` instead)

However, because the behavior of inline_svg is effectively unchanged, there may be no need to deprecate it at all!

How it works

Under the hood we have a new method, with_asset_finder, which is passed a finder class (or nil) and a block. Calls to render_inline_svg within the block will use the finder passed to with_asset_finder.

This allows us to temporarily change InlineSvg.configuration.asset_finder for the duration of the block, which is how we're able to keep the existing inline_svg helper backwards-compatible.

Disclaimer: I have limited familiarity with the internal guts of this gem, so it's entirely possible that altering InlineSvg.configuration.asset_finder as I've done has side-effects on other parts of the gem's functionality. All the tests still pass, so I'm hoping there are no side-effects I've overlooked 🤞

I'll add tests for everything if this general approach is approved 👍

[jamesmartin]

@kylefox thanks for pulling this together. I like this approach as a stepping stone to completely deprecating the inline_svg method (at least publicly) in v2.0.

[kylefox]

@jamesmartin I updated the README and added tests for the new helpers.

(Actually, I didn't add any new tests. I extracted the existing examples into shared examples and used them to test all three helpers. Open to suggestions if you have other ideas for how to test this).

One thing I'm curious about in general — what other use-cases have you seen for using a custom asset_finder, besides webpacker support?

[jamesmartin]

One thing I'm curious about in general — what other use-cases have you seen for using a custom asset_finder, besides webpacker support?

One concrete use-case is in situations where development versus production differ. For example, when caching all SVG assets at Rails boot time it's useful to implement a separate asset finder for use in development mode to allow dynamic reloading from disk.