Syntax lookup: let \"" (rescript-lang#242)

* Syntax Lookup: let binding invalid

* Update misc_docs/syntax/language_let_invalid.mdx

Co-authored-by: Patrick Ecker <[email protected]>

Author: kevanstannard

misc_docs/syntax/language_let_invalid.mdx

@@ -0,0 +1,35 @@
+---
+id: "let-binding-invalid"
+keywords: ["let", "variable"]
+name: "let \\\"\""
+summary: "This is the `let binding` for invalid identifier names."
+category: "languageconstructs"
+---
+
+`let \""` is used to bind values to names that are invalid in ReScript, including capitalized names, names with special characters, and reserved keywords.
+
+**Warning: ** This escape hatch is almost exclusively used for interop with JavaScript. If you abuse this, many of the compiler guarantees will go away.
+
+### Example
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+let \"ComponentName" = "Header"
+let \"min-width" = 640
+let \"exception" = true
+```
+
+```js
+var ComponentName = "Header";
+var min$width = 640;
+var exception = true;
+```
+
+</CodeTab>
+
+### References
+
+* [Use Illegal Identifier Names](/docs/manual/latest/use-illegal-identifier-names)
+* [Let binding](/docs/manual/latest/let-binding)
+* [Reserved Keywords](/docs/manual/latest/reserved-keywords)