Add syntax highlighting for SML

Converted from
https://github.com/seanjames777/SML-Language-Definition/blob/master/sml.tmLanguage
and removed `ml` filetype to fix an (assumed) possible conflict with
existing OCaml support.

assets/README.md

@@ -34,6 +34,7 @@ The following files have been manually modified after converting from a `.tmLang
 * `Dart.sublime-syntax` => removed `#regex.dart` include.
 * `INI.sublime-syntax` => added `.hgrc`, `hgrc`, and `desktop` file types.
 * `Org mode.sublime-syntax` => removed `task` file type.
+* `SML.sublime_syntax` => removed `ml` file type.
 
 ### Non-submodule additions
 
@@ -42,3 +43,8 @@ The following files have been manually modified after converting from a `.tmLang
 * `Rego.sublime-syntax` has been added manually from https://github.com/open-policy-agent/opa/blob/master/misc/syntax/sublime/rego.sublime-syntax
    as it is not kept in a standalone repository. The file is generated from
    https://github.com/open-policy-agent/opa/blob/master/misc/syntax/textmate/Rego.tmLanguage
+* `SML.sublime_syntax` has been added manually from
+   https://github.com/seanjames777/SML-Language-Definitiona as it is not
+   kept in a standalone repository. The file generated is from
+   https://github.com/seanjames777/SML-Language-Definition/blob/master/sml.tmLanguage
+

assets/syntaxes/sml.sublime-syntax

@@ -0,0 +1,33 @@
+%YAML 1.2
+---
+# http://www.sublimetext.com/docs/3/syntax.html
+name: SML
+file_extensions:
+  - sml
+  - cm
+  - sig
+scope: source.sml
+contexts:
+  main:
+    - match: \b(EQUAL|GREATER|LESS|NONE|SOME|abstraction|abstype|and|andalso|array|as|before|bool|case|char|datatype|do|else|end|eqtype|exception|exn|false|fn|fun|functor|handle|if|in|include|infix|infixr|int|let|list|local|nil|nonfix|not|o|of|op|open|option|orelse|overload|print|raise|real|rec|ref|sharing|sig|signature|string|struct|structure|substring|then|true|type|unit|val|vector|where|while|with|withtype|word)\b
+      comment: SML keywords
+      scope: keyword.source.sml
+    - match: '\b[0-9]+\b'
+      comment: Numeric constants
+      scope: constant.numeric.sml
+    - match: '\b[A-Z]([A-z0-9]*)\b'
+      comment: Built in types
+      scope: support.type.sml
+    - match: '"(\\"|[^"])*"'
+      comment: String constant
+      scope: string.sml
+    - match: \(\*
+      comment: Comment
+      push:
+        - meta_scope: comment.sml
+        - match: \*\)
+          pop: true
+    - match: '(\(\)|=>|::|\[\]|->|:>)'
+      comment: Character
+      scope: constant.character.sml
+