Improve heuristic for XML property lists

[Alhadis]

Description

Some XML property lists lack a <!DOCTYPE[^1] and/or XML signature[^2], and neither omission impacts the file's well-formedness (according to Xcode). Apparently, the only hard requirement is well-formed—though not necessarily valid—XML syntax.

This PR amends the heuristic for XML property list so it matches .plist files that:

1. contain a line comprised wholly of <plist> (with or without a version attribute),
2. contain an XML signature, which is invalid syntax in OpenStep property lists, or
3. contain a line beginning with <!DOCTYPE plist.

Case 1 could extend to other data-type tags as well (especially <array> and <dict>), but the grammar we're using highlights such tags as invalid when they appear outside of a <plist> element:

Example 1: <array>

<array>
	<string>/usr/local/bin</string>
	<string>/usr/bin</string>
	<string>/bin</string>
	<string>/usr/sbin</string>
	<string>/sbin</string>
</array>

Example 2: <dict>

<dict>
	<key>kubernetes.docker.internal</key>
	<string>127.0.0.1</string>
	<key>localhost</key>
	<array>
		<string>127.0.0.1</string>
		<string>::1</string>
	</array>
	<key>broadcasthost</key>
	<string>255.255.255.255</string>
</dict>

Checklist

(Template removed as it doesn't apply)

Sample files

File	Source	License
ff-man.plist	Apple's fork of man(1)	GPL v2
man.plist
info.min.plist	Copied from local installation of cool-retro-term3	GPL v3+

Footnotes

1. https://github.com/apple-oss-distributions/man/blob/54b31db2a21b33fe587826dbce22bac7ac775dce/ff-man.plist

2. https://github.com/apple-oss-distributions/man/blob/54b31db2a21b33fe587826dbce22bac7ac775dce/man.plist

3. Specifically, /Applications/cool-retro-term.app/Contents/Info.plist. Newlines were stripped in order to assert that XML declaration matching is working.

[lildude]

Makes sense to me.