Ignore length specifications after linebreaks

In LaTeX line breaks in matrix elements
have an optional argument that controls
the vertical spacing between the current
and the next line. For example,

{\begin{aligned}a\\[6pt] b\end{aligned}}

generates a 6pt space.

This was never implemented and ignored by
mathoid.
The native MathML implementation is unaware
of the optional argument and just prints it
to the next line.

* This commit ignores the input on the grammar
side.

Bug: T375295
Change-Id: I08996129beca9ad852d23f28d7136c982707aea1

src/WikiTexVC/parser.pegjs

@@ -384,8 +384,33 @@ FUN_AR1opt
 NEXT_CELL
  = "&" _
 
+LATEX_LENGTH
+  = LATEX_SIGN? LATEX_NUMBER LATEX_UNIT
+
+LATEX_SIGN
+  = [+-]
+
+LATEX_NUMBER
+  = literal_mn+ "."? literal_mn*
+  / "." literal_mn+
+
+LATEX_UNIT
+  = "pt"
+  / "cm"
+  / "mm"
+  / "in"
+  / "em"
+  / "ex"
+  / "bp"
+  / "pc"
+  / "dd"
+  / "cc"
+  / "sp"
+  / "nd"
+  / "nc"
+
 NEXT_ROW
- = "\\\\" _
+ = "\\\\" ("[" LATEX_LENGTH "]")? _
 
 BEGIN
  = "\\begin" _

tests/phpunit/unit/WikiTexVC/ApiTest.php

@@ -124,6 +124,20 @@ class ApiTest extends MediaWikiUnitTestCase {
 				'mhchem_required' => true,
 				'status' => 'C'
 			],
+			[
+				'in' => '{\\begin{aligned}a\\\\b\\end{aligned}}',
+				'ams_required' => true,
+			],
+			[
+				'in' => '{\\begin{aligned}a\\\\[6pt] b\\end{aligned}}',
+				'output' => '{\\begin{aligned}a\\\\b\\end{aligned}}',
+				'ams_required' => true,
+			],
+			[
+				'in' => '{\\begin{aligned}a\\\\[-3.4mm] b\\end{aligned}}',
+				'output' => '{\\begin{aligned}a\\\\b\\end{aligned}}',
+				'ams_required' => true,
+			],
 		];
 		foreach ( $testCases as $case ) {
 			yield $case['in'] => [ $case ];