feat: add --show-dotfiles

[bcullman]

On Unix-like systems, "hidden" usually means a name starts with (.). On Windows, there is also a separate filesystem hidden attribute. Because of that, -a is often a reasonable way to show dotfiles on Unix-like systems, but can be much noisier on Windows.

Examples:

Unix-like:

eza -l
-rw-r--r-- file.txt

eza -la
lrwxrwxrwx .aws -> /some/shared/path
-rw-r--r-- .bashrc
-rw-r--r-- file.txt

Windows:

eza -l
d-r-- Documents
d---- source

eza -la
d---- .dotnet
d---- .nuget
l--hs 'Application Data' -> C:\Users\name\AppData\Roaming
l--hs Cookies -> C:\Users\name\AppData\Local\Microsoft\Windows\INetCookies
-a-h- NTUSER.DAT
---h- XORXOR659040683.doc
---h- XORXOR779122472.db
---h- ZZZZZ2148549623.pdf
---h- ZZZZZ3872883360.sql
d-r-- Documents
d---- source

This PR adds --show-dotfiles as a narrower option:

• shows dot-prefixed entries on all platforms
• on Windows, also shows _-prefixed entries, matching eza's existing hidden-by-name behavior
• does not show Windows hidden-attribute entries
• does not change -a or -aa

Unix-like:

eza -l
-rw-r--r-- file.txt

eza -l --show-dotfiles
lrwxrwxrwx .aws -> /some/shared/path
-rw-r--r-- .bashrc
-rw-r--r-- file.txt

Windows:

eza -l
d-r-- Documents
d---- source

eza -l --show-dotfiles
d---- .dotnet
d---- .nuget
d-r-- Documents
d---- source

This helps with a common Windows case where -a is too broad because it also reveals hidden-attribute files and Windows hidden links/aliases.

Some entries in a Windows home directory are also hidden links/aliases rather than regular files or directories. Examples include entries such as Local Settings and SendTo. Links do not have to be hidden in general, but many of these Windows profile links are.

Those entries are intentionally out of scope for this PR. This change focuses only on showing dotfiles without also enabling the broader Windows hidden-attribute view. A sister PR will follow to add a way to show those hidden Windows links/aliases as well.

This is one of the PRs needed to address #1720 #1768