#467: generate summary XML in docs command

[nacNAC333]

Fixes #467

Adds summary XML file generation to the docs command. After generating HTML documentation, the command now also produces a summary.xml file in <eodoc> format listing all packages and objects.

Changes:

• Add xmlEscape() helper for safe XML entity escaping
• Generate summary.xml with <eodoc> root, <package> and <object> children
• Include packages and objects count attributes on root element

[yegor256]

@nacNAC333 Thanks for your contribution! Unfortunately, we can't merge it without tests. Every feature we add and every bug we fix must be accompanied by at least one new test that confirms that the code was not working before. Without such a test, how can we know that what you commit actually works?

[Favixx]

docs command crashes — rel is not defined

Running the docs test suite on this branch (test/commands/test_docs.js) fails: line 178 calls rel(summary), but rel is not defined or imported anywhere in src/commands/docs.js.

Error generating documentation: ReferenceError: rel is not defined
    at src/commands/docs.js:178:15
    at elapsed (src/elapsed.js:23:10)
    at module.exports [as docs] (src/commands/docs.js:124:10)

The exception is caught at line 149, so execution aborts before summary.xml is written — the feature never produces output, and the generates HTML files for files and packages test fails.

Suggested follow-ups:

1. Replace rel(summary) with something defined, e.g. path.relative(output, summary).
2. The nested O(packages × xmirs) loop re-derives data that packages_info already groups — consider reusing it instead of recomputing path.relative per xmir per package.
3. Please add a test asserting summary.xml is created with the expected packages/objects counts — none exists currently, which is why this regression slipped through.

[nacNAC333]

@yegor256 Test added in the second commit — generates summary XML with correct package and object counts verifies the file is created and checks package/object counts, package names, and object names. Also rebased on latest master.