Hare: add word boundaries to as and is operators - #3309

Fixes #3308.

Summary

The Hare lexer listed the operators as and is alongside the
symbol operators =>, .. and ... in a single words()
rule without word boundaries. Because as and is consist of
word characters, the rule matched them as a prefix of any identifier
starting with those letters, e.g. isfile was split into an
Operator is followed by Name file.

This change adds word boundaries to the word-like operators only, and
keeps the symbol operators (=>, .., ...) matching
unconditionally, as those must also match next to word characters
(e.g. i < len && arr[i].. style syntax).

Changes

• pygments/lexers/hare.py: split the operator rule so as/is use
  prefix/suffix word boundaries while =>, .., ... stay
  unbounded.
• tests/snippets/hare/test_keyword_operators.txt: regression snippet
  covering identifiers such as isfile, ascii, is_valid and the
  standalone as/is operators.
• CHANGES: changelog entry under 2.22.0.

Tests

Verified locally with the full test suite:

• pytest tests/ → 5331 passed, 16 skipped
• The new regression snippet fails without the lexer change and passes
  with it.