patch 9.1.1268: filetype: dax files are not recognized

Problem:  filetype: dax files are not recognized
Solution: detect "*.dax" as dax filetype, include dax filetype and
          syntax plugin (Anarion Dunedain)

Data Analysis Expressions (DAX) is a formula expression language used in
Analysis Services, Power BI, and Power Pivot in Excel. DAX formulas
include functions, operators, and values to perform advanced
calculations and queries on data in related tables and columns in
tabular data models.

DAX language overview:
- https://learn.microsoft.com/en-us/dax/dax-overview

closes: #17035

Signed-off-by: Anarion Dunedain <anarion80@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/dax.vim

@@ -0,0 +1,16 @@
+" Vim filetype plugin
+" Language: Data Analysis Expressions (DAX)
+" Maintainer: Anarion Dunedain <anarion80@gmail.com>
+" Last Change: 2025 Apr 2
+
+if exists('b:did_ftplugin')
+  finish
+endif
+
+let b:did_ftplugin = 1
+
+setlocal comments=sO:*\ -,mO:*\ \ ,exO:*/,s1:/*,mb:*,ex:*/,://
+
+setlocal commentstring=//\ %s
+
+let b:undo_ftplugin = 'setl com< cms<'