Add symbol double square brackets (#1947)

* Add symbol double square brackets

* Update name for double square brackets, add macros and documentation

docs/support_table.md

@@ -570,6 +570,7 @@ table td {
 |\limsup|$\limsup$||
 |\ll|$\ll$||
 |\llap|${=}\llap{/\,}$|`{=}\llap{/\,}`|
+|\llbracket|$\llbracket$||
 |\llcorner|$\llcorner$||
 |\Lleftarrow|$\Lleftarrow$||
 |\lll|$\lll$||
@@ -877,6 +878,7 @@ table td {
 |\rotatebox|<span style="color:firebrick;">Not supported</span>|[Issue #681](https://github.com/KaTeX/KaTeX/issues/681)|
 |\rparen|$\rparen$||
 |\rq|$\rq$||
+|\rrbracket|$\rrbracket$||
 |\Rrightarrow|$\Rrightarrow$||
 |\Rsh|$\Rsh$||
 |\rtimes|$\rtimes$||

docs/supported.md

@@ -56,7 +56,7 @@ See also [letters](#letters)
 |$\vert$ <code>&#124;</code> |$\vert$ `\vert` |$┌ ┐$ `┌ ┐`|$\ulcorner \urcorner$ `\ulcorner`<br>$~~~~$`\urcorner`  |$\Downarrow$ `\Downarrow`
 |$\Vert$ <code>&#92;&#124;</code> |$\Vert$ `\Vert` |$└ ┘$ `└ ┘`|$\llcorner \lrcorner$ `\llcorner`<br>$~~~~$`\lrcorner`  |$\Updownarrow$ `\Updownarrow`
 |$\lvert~\rvert$ `\lvert`<br>$~~~~$`\rvert`|$\lVert~\rVert$ `\lVert`<br>$~~~~~$`\rVert` |`\left.`|  `\right.` |$\backslash$ `\backslash`
-|$\lang~\rang$ `\lang`<br>$~~~~$`\rang`|$\lt~\gt$ `\lt \gt`|$⟦~⟧$ `⟦ ⟧`|
+|$\lang~\rang$ `\lang`<br>$~~~~$`\rang`|$\lt~\gt$ `\lt \gt`|$⟦~⟧$ `⟦ ⟧`|$\llbracket~\rrbracket$ `\llbracket`<br>$~~~~$`\rrbracket`|
 
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