Implements comments for fish_plugins

[soraxas]

This PR aims to implement a simple comment system to address #635.

Commit 11eb085 had linked #524 of deprecating fishfile for fish_plugins, which is nice but comments supports are dropped, perheps due to the previous filefile's complexity.

This PR aims to introduce a simple and easy to maintain comment system back to fish_plugins (partly based on ae770fc). Currently, this commit works on both full line comments and inline comments (where there should be at least one whitespace before the first # character), using only built-in fish-shell string regex.

Explaination:

The following

string match --regex -- '^[^#\s]+' <$fish_plugins

can essentially strip away any full or inline comments as it captures from the beginning of the line to the first non-whitespace non-# character.

Then, in the logic of writing back to $fish_plugins, we first use

# match line comments, active plugins (+ optional inline comments) in fish_plugins
set --local -- new_content (string match --regex -- '^\s*#.*$|^(?:'(
    string join -- '|' $_fisher_plugins)')(?:\s+#.*)?$' <$fish_plugins)

to construct a regex with the installed active plugins to filter out the content of $fish_plugins. The regex only match fullline comments ('^\s*#.*$), active plugins, and an optional inline-comment capture group ((?:\s+#.*)?) after the active plugins. Note that we had checked $_fisher_plugins[1] to be defined so the string join should always has at least one item.

Finally,

# inverse match active plugins against the (comment stripped) $new_content
set --append -- new_content (string match --regex --invert -- '^(?:'(
    string join -- '|' (string match --regex -- '^[^#\s]+' $new_content))')$' $_fisher_plugins)

first strips away any full/inline comments from $new_content, then performs an exact inverse match so that any newly installed plugins in $_fisher_plugins that are currently not present in $fish_plugins would be added to $new_content. As an added bonus, the contents and comments in $fish_plugins will be order-preserved with this approach, and newly installed plugins are appended at the end of the file.

[jorgebucaran]

Thank you for creating a new POC, but I just closed #635 stating that I will not be adding comment support for now.

BTW, I think you forgot to string escape in your implementation to encode plugin names.

[jorgebucaran]

@soraxas Renabling support for top-level .fish files in this PR was uncalled for. Please take a break.