Add more support for fs.FS in template parsing

[doug-threatmate]

Proposed changes

This adds a new LoadHelperFileFunction option to Options, which allows the user to specify a function to use to load a helper file (since the current one does not respect the catalog yet). This can be used to fully implement template parsing using a custom fs.FS without ever needing to touch the file system.

This adds more support for fs.FS in the disk catalog for completeness. This fixes some places where direct os file-related calls were being made to use the catalog interface instead.

Note that the JavaScript compiler still does not work in any context where the pkg/js/libs/fs package is used. In particular, the ReadFilesFromDir function is hard-coded to use the os package and not respect the catalog.

Checklist

• Pull request is created against the dev branch
• All checks passed (lint, unit/integration/regression tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[doug-threatmate]

@tarunKoyalwar okay, I think that I've done what you wanted. This implementation is sufficient for my needs. Long-term, I'd love to see more true catalog support, but I think that that will mean fully deprecating a lot of weird local path things that seem to have been carried forward from the old days.

[doug-threatmate]

For usage, here's what I'm doing (and it works great):

	ne.Options().LoadHelperFileFunction = func(helperFile, templatePath string, catalog catalog.Catalog) (io.ReadCloser, error) {
		return catalog.OpenFile(helperFile)
	}

Hopefully when we refactor all of the old file system code over the next year or so, we can just use the catalog the way that it was designed to be used (like this) 😂

[doug-threatmate]

@tarunKoyalwar do you need anything else from me for this PR?

[doug-threatmate]

@tarunKoyalwar anything else you need from me?

[Mzack9999]

@doug-threatmate I am unable to push in your branch, so I'd recommend the following changes:

• Use a generic function signature

type LoadHelperFileFunc func(helperFile, templatePath string, catalog catalog.Catalog) (io.ReadCloser, error)

• Replace it in options as follows:

type Options struct {
...
// LoadHelperFileFunc is a function that will be used to execute LoadHelperFile.
// If none is provided, then the default implementation will be used.
LoadHelperFileFunc LoadHelperFileFunc
...
}

Then just add the following in the:

func (options *Options) LoadHelperFile(helperFile, templatePath string, catalog catalog.Catalog) (io.ReadCloser, error) {
if options.LoadHelperFileFunc != nil {
		return options.LoadHelperFileFunc(helperFile, templatePath, catalog)
	}
// same code as before

[doug-threatmate]

Updated per @Mzack9999's request