Add Roblox .rbxmx as XML

[gords2]

Add .rbxmx and .rbxlx to the XML language.

Description

.rbxmx (Roblox model) and .rbxlx (Roblox place) are the XML-based serialization formats produced by Roblox Studio and the wider Roblox tooling ecosystem (Rojo, etc.). Both are well-formed XML documents rooted in a <roblox …> element with an xsi:noNamespaceSchemaLocation pointing at roblox.xsd, so they classify naturally as XML.

This PR:

• Adds .rbxlx and .rbxmx to the XML entry in languages.yml, kept in case-sensitive alphabetical order (between .qhelp and .rdf).
• Adds one real-world sample for each extension.

Neither extension is currently associated with any other language, so no heuristic is required.

Resolves #7928.

Checklist:

• I am adding a new extension to a language.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.rbxmx+roblox
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.rbxlx+roblox
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • samples/XML/roblox-model.rbxmx — from rojo-rbx/rbx-test-files models/three-unique-parts/xml.rbxmx
      • samples/XML/roblox-baseplate.rbxlx — from rojo-rbx/rbx-test-files places/baseplate-413/xml.rbxlx
    • Sample license(s): MIT (both samples are from rojo-rbx/rbx-test-files, which is MIT-licensed)
  • N/A I have included a change to the heuristics to distinguish my language from others using the same extension. (Not applicable — neither extension is shared with another language.)

[Alhadis]

Usage for the .rbxlx extension is too thin to be eligible for addition; there's currently only 108 such files indexed on GitHub. The .rbxmx extension, OTOH, clearly checks out (~46.2k search results for that extension).

In addition, the size of the samples you've added are, uh, kind of huge (and they'll slow the classifier down needlessly). Could you cull the .rbxmx sample down to, say, ~100 lines or so? (Making sure the source remains well-formed XML, though).

You'll also need to remove the .rbxlx extension from this PR (and thus also its sample), owing to the scant in-the-wild usage.

Both are well-formed XML documents

😉 Technically, a well-formed XML document includes an XML declaration (<?xml version="1.0"?>), which incidentally is the reason these files aren't being recognised by Linguist's [XML header] strategy.

[gords2]

Pushed 71c75be addressing the review:

• Dropped the .rbxlx extension and its sample (roblox-baseplate.rbxlx) given the thin in-the-wild usage.
• Cut roblox-model.rbxmx from 246 → 82 lines (one Part), still well-formed XML.

Good call on the XML-declaration point — and that is precisely why the extension mapping is needed here: Roblox Studio writes these files without an <?xml ?> prolog, so the XML-header strategy never fires. Left the sample prolog-less to stay representative of real-world .rbxmx output.

[Alhadis]

Excellent work!