don't treat :foo as a directive

[Rich-Harris]

small tweak to #1242 — also fixes the over-specific regex.

Also disallows ref:foo='bar' etc — unlike other directives, refs don't have a value

[codecov-io]

Codecov Report

Merging #1245 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1245      +/-   ##
==========================================
- Coverage    91.9%   91.89%   -0.01%     
==========================================
  Files         126      126              
  Lines        4557     4554       -3     
  Branches     1486     1485       -1     
==========================================
- Hits         4188     4185       -3     
  Misses        153      153              
  Partials      216      216

Impacted Files	Coverage Δ
src/parse/read/directives.ts	86.44% <100%> (-0.66%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6f866bb...39694c3. Read the comment docs.