Add support for Elvish (https://elv.sh).

Usage in the wild:
https://github.com/search?q=extension%3Aelv&type=Code&ref=advsearch&l=&l=. There
are a couple of false positives, but the vast majority of the results are Elvish
scripts.

Code sample is taken from Elvish's standard library:
https://github.com/elves/elvish/blob/master/pkg/mods/epm/epm.elv, license BSD
2-clause.

.gitmodules

@@ -425,6 +425,9 @@
 [submodule "vendor/grammars/elixir-tmbundle"]
 	path = vendor/grammars/elixir-tmbundle
 	url = https://github.com/elixir-lang/elixir-tmbundle
+[submodule "vendor/grammars/elvish"]
+	path = vendor/grammars/elvish
+	url = https://github.com/elves/elvish.git
 [submodule "vendor/grammars/erlang.tmbundle"]
 	path = vendor/grammars/erlang.tmbundle
 	url = https://github.com/textmate/erlang.tmbundle

grammars.yml

@@ -348,6 +348,8 @@ vendor/grammars/elixir-tmbundle:
 - source.elixir
 - text.elixir
 - text.html.elixir
+vendor/grammars/elvish:
+- source.elvish
 vendor/grammars/erlang.tmbundle:
 - source.erlang
 - text.html.erlang.yaws

lib/linguist/languages.yml

@@ -1700,6 +1700,16 @@ Elm:
   codemirror_mode: elm
   codemirror_mime_type: text/x-elm
   language_id: 101
+Elvish:
+  type: programming
+  ace_mode: text
+  extensions:
+  - ".elv"
+  interpreters:
+  - elvish
+  tm_scope: source.elvish
+  color: "#55BB55"
+  language_id: 570996448
 Emacs Lisp:
   type: programming
   tm_scope: source.emacs.lisp