Often, division/regexp ambiguity doesn't matter in JS because the next
token is not a slash.

For example, in

  <script>var global{{if .InitVal}} = {{.InitVal}}{{end}}</script>

When there is an initial value, the {{if}} ends with jsCtxDivOp
since a '/' following {{.InitVal}} would be a division operator.
When there is none, the empty {{else}} branch ends with jsCtxRegexp
since a '/' would start a regular expression.  A '/' could result
in a valid program if it were on a new line to allow semicolon
insertion to terminate the VarDeclaration.

There is no '/' though, so we can ignore the ambiguity.

There are cases where a missing semi can result in ambiguity that
we should report.

  <script>
  {{if .X}}var x = {{.X}}{{end}}
  /...{{.Y}}
  </script>

where ... could be /foo/.test(bar) or /divisor.  Disambiguating in
this case is hard and is required to sanitize {{.Y}}.

Note, that in the case where there is a '/' in the script tail but it
is not followed by any interpolation, we already don't care.  So we
are already tolerant of

<script>{{if .X}}var x = {{.X}}{{end}}/a-bunch-of-text</script>

because tJS checks for </script> before looking in /a-bunch-of-text.

This CL
- Adds a jsCtx value: jsCtxUnknown
- Changes joinContext to join contexts that only differ by jsCtx.
- Changes tJS to return an error when a '/' is seen in jsCtxUnknown.
- Adds tests for both the happy and sad cases.

R=nigeltao
CC=golang-dev
https://golang.org/cl/4956077