Gate (at least) Bash support behind a feature flag

[Xophmeister]

Is your feature request related to a problem? Please describe.
We have seen a number of times that building tree-sitter-bash fails as it references a (somehow) invalid ref on Savannah (GNU's git service, which hosts the Bash repository). The offending commit does appear to exist, so it's not clear why this occurs.

In the past, we've managed to resolve this by bumping the tree-sitter-bash version. However, IIRC, we have reached the point where this is no longer possible; newer versions use TS 0.21 (see #684). It's therefore possible that this problem could reoccur and there'd be no simple fix.

Describe the solution you'd like
Put Bash support behind a feature gate, which is disabled by default.

Describe alternatives you've considered
Remove Bash support completely, until we can move to a newer version.

Additional context
topiary-queries already uses feature flags; I believe we'd just have to remove Bash from the default features to achieve this.

We're singling out Bash because it is unstable, but perhaps there is also an argument to be had for gating all experimental languages behind an off-by-default feature flag.