feat(markdown-and-latex): add markmap.nvim (#229)

* Added: Markmap.nvim

* Let's not give the html_output option, so it's detected automatically for windows too.

* Use `npm` instead of `yarn` because more AstroNvim users have it

---------

Co-authored-by: Micah Halter <micah@mehalter.com>

lua/astrocommunity/markdown-and-latex/markmap/README.md

@@ -0,0 +1,7 @@
+# markmap.nvim
+
+**Repository:** <https://github.com/Zeioth/markmap.nvim>
+
+Visualize your Markdown as mindmaps with markmap.
+
+**Note:** This will install `markmap-cli` globally on your system with `npm`.

lua/astrocommunity/markdown-and-latex/markmap/markmap-nvim.lua

@@ -0,0 +1,10 @@
+--  [markdown markmap]
+--  https://github.com/Zeioth/markmap.nvim
+return {
+  "Zeioth/markmap.nvim",
+  build = "npm install -g markmap-cli",
+  cmd = { "MarkmapOpen", "MarkmapSave", "MarkmapWatch", "MarkmapWatchStop" },
+  opts = {
+    hide_toolbar = "false",
+  },
+}