enh(nix): major improvements to grammar (#4140)

* nix: add function type, import regex library

* nix: update keywords

- Add a bunch of new keywords
- List all `builtins` explicitly
- Sort alphabetically, to make it easier to spot missing builtins
- Remove `add`, not a real builtin
- Move `or` to `keywords`, this is not a `literal`

* nix: fix string handling

- The different string variants have different modes of escapes.
  Split `STRING.contains` into their different variants to reflect this.
- Add escape logic for `'''`
- Add escape logic for backslash escaped characters.

* nix: handle path, lookup paths, and operators

* nix: handle markdown comments

Also ensure comments are handled before any other expressions

* nix: handle REPL keywords

* nix: handle basic function params

* nix: better parsing for attrsets

* nix: update tests

* CHANGES.md: update with changes to nix

Author: h7x4

CHANGES.md

@@ -17,13 +17,26 @@ Core Grammars:
 - enh(csharp) add Contextual keywords `file`, `args`, `dynamic`, `record`, `required` and `scoped` [Alvin Joy][]
 - enh(lua) add 'pluto' as an alias [Sainan]
 - enh(bash) add reserved keywords `time` and `coproc` [Álvaro Mondéjar][]
+- enh(nix) update keywords [h7x4][]
+- enh(nix) support paths [h7x4][]
+- enh(nix) support lookup paths [h7x4][]
+- enh(nix) support operators [h7x4][]
+- enh(nix) support REPL keywords [h7x4][]
+- enh(nix) support markdown comments [h7x4][]
+- enh(nix) support basic function params [h7x4][]
+- enh(nix) better parsing of attrsets [h7x4][]
 - fix(c) - Fixed hex numbers with decimals  [Dxuian]
 - fix(typescript) - Fixedoptional property not highlighted correctly  [Dxuian]
 - fix(ruby) - fix `|=` operator false positives (as block arguments) [Aboobacker MK]
 - fix(sql) - Fixed sql primary key and foreign key spacing issue   [Dxuian]
 - fix(cpp) added flat_set and flat_map as a part of cpp 23 version [Lavan]
 - fix(yaml) - Fixed special chars in yaml   [Dxuian]
 - fix(basic) - Fixed closing quotation marks not required for a PRINT statement [Somya]
+- fix(nix) remove `add` builtin [h7x4][]
+- fix(nix) mark `or` as builtin instead of literal [h7x4][]
+- fix(nix) handle `'''` string escapes [h7x4][]
+- fix(nix) handle backslash string escapes [h7x4][]
+- fix(nix) don't mix escapes for `"` and `''` strings [h7x4][]
 - fix(swift) - Fixed syntax highlighting for class func/var declarations [guuido]
 
 New Grammars: