feat(syntax): basic support for ddot

refer: #3091

Author: lervag

autoload/vimtex/syntax/core.vim

@@ -1443,6 +1443,27 @@ let s:alphabet_map = {
       \   ['Z', 'Ż'],
       \   ['z', 'ż'],
       \ ],
+      \ 'ddot': [
+      \   ['A', 'Ä'],
+      \   ['a', 'ä'],
+      \   ['E', 'Ë'],
+      \   ['e', 'ë'],
+      \   ['H', 'Ḧ'],
+      \   ['h', 'ḧ'],
+      \   ['I', 'Ï'],
+      \   ['i', 'ï'],
+      \   ['O', 'Ö'],
+      \   ['o', 'ö'],
+      \   ['t', 'ẗ'],
+      \   ['U', 'Ü'],
+      \   ['u', 'ü'],
+      \   ['W', 'Ẅ'],
+      \   ['w', 'ẅ'],
+      \   ['X', 'Ẍ'],
+      \   ['x', 'ẍ'],
+      \   ['Y', 'Ÿ'],
+      \   ['y', 'ÿ'],
+      \ ],
       \ 'hat': [
       \   ['a', 'â'],
       \   ['A', 'Â'],
@@ -2024,6 +2045,7 @@ function! s:match_math_symbols() abort " {{{1
         \ ['bar', 'bar'],
         \ ['hat', 'hat'],
         \ ['dot', 'dot'],
+        \ ['ddot', 'ddot'],
         \ ['\%(var\)\?math\%(bb\%(b\|m\%(ss\|tt\)\?\)\?\|ds\)', 'double'],
         \ ['mathfrak', 'fraktur'],
         \ ['math\%(scr\|cal\)', 'script'],

test/test-syntax/test-conceal.tex

@@ -159,4 +159,9 @@ \subsubsection{testing}
 $\{x\}$
 $\left\{x\right\}$
 
+\[
+  \dot a
+  \ddot a
+\]
+
 \end{document}