ESQL: Break up antlr lexer

This breaks the antlr lexer into a few files rather than one giant file.
It's much easier to read them this way.

Author: nik9000

x-pack/plugin/esql/build.gradle

@@ -205,6 +205,7 @@ tasks.register("regenLexer", JavaExec) {
     '-package', 'org.elasticsearch.xpack.esql.parser',
     '-listener',
     '-visitor',
+    '-lib', "${file(grammarPath)}/lexer",
     '-o', outputPath,
     "${file(grammarPath)}/EsqlBaseLexer.g4"
 }