feat(wpcarro/simple-select): support tokenizer for query language

Support a tokenizer for a query language that looks like:

```
-fname:/W.*m/ lname:"Von Carroll"
```

Parser otw...

Change-Id: I2badf14a41313ca2f75dec20adbcf9031b22ab83
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5338
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/scratch/simple-select/scanner.py

@@ -2,15 +2,15 @@
 # scanner/lexer needs are peek and advance; other functions (e.g. match) are
 # nice-to-haves.
 class Scanner(object):
-  def __init__(self, source):
+  def __init__(self, chars):
     self.i = 0
-    self.source = source
+    self.chars = chars
 
   def exhausted(self):
-    return self.i >= len(self.source)
+    return self.i >= len(self.chars)
 
   def peek(self, n=0):
-    return self.source[self.i + n] if self.i + n < len(self.source) else '\0'
+    return self.chars[self.i + n] if self.i in range(0, len(self.chars)) else '\0'
 
   def advance(self):
     result = self.peek()