Add $es3 token to SkSL.

This will become a modifier in followup CLs; the presence of this
modifier will be used to indicate intrinsics which are ES3-specific
(and therefore, not allowed in user code under typical circumstances).

Change-Id: I968309e33ad80f9382bea8dc2f8fd2204b378777
Bug: skia:12202
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/439056
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

src/sksl/SkSLLexer.h

@@ -50,6 +50,7 @@ struct Token {
         TK_HIGHP,
         TK_MEDIUMP,
         TK_LOWP,
+        TK_ES3,
         TK_IDENTIFIER,
         TK_DIRECTIVE,
         TK_LPAREN,

src/sksl/generated/sksl_public.dehydrated.sksl

@@ -1327,7 +1327,7 @@ static uint8_t SKSL_INCLUDE_sksl_public[] = {4,2,
 46,139,1,2,
 1,
 45,
-55,138,1,0,3,0,1,2,49,
+55,138,1,0,3,0,1,2,50,
 27,
 46,123,0,155,0,2,
 45,
@@ -1344,7 +1344,7 @@ static uint8_t SKSL_INCLUDE_sksl_public[] = {4,2,
 46,142,1,2,
 1,
 45,
-55,141,1,0,3,0,1,2,49,
+55,141,1,0,3,0,1,2,50,
 27,
 46,115,0,147,0,2,
 45,

src/sksl/lex/sksl.lex

@@ -42,6 +42,7 @@ LAYOUT         = "layout"
 HIGHP          = "highp"
 MEDIUMP        = "mediump"
 LOWP           = "lowp"
+ES3            = "$es3"
 IDENTIFIER     = [a-zA-Z_$]([0-9]|[a-zA-Z_$])*
 DIRECTIVE      = #[a-zA-Z_$]([0-9]|[a-zA-Z_$])*
 LPAREN         = "("