v.pref: prevent overriding backend (fix #21758)

[gchumillas]

Fixes #21758

Related to PR #23797

[huly-for-github]

Connected to Huly®: V_0.6-22894

[JalonSolov]

Is this really a good thing? There are multiple JS types for a reason... the first being that you may want to run the code in a browser, not node. This looks like changes things to only output for node.

[gchumillas]

Is this really a good thing? There are multiple JS types for a reason... the first being that you may want to run the code in a browser, not node. This looks like changes things to only output for node.

This PR only prevents the manually entered -backend flag from being overridden. For example:

// -backend is "js" by default
v main.v -o main.js

// don't override -backend, as it has been manually entered
v -backend js_browser main.v -o main.js

The default value was established here (line 1054):
https://github.com/vlang/v/pull/23797/files#diff-e706d00d8dda53e89b46391f9de496dc45f5efc1fb9776c3d230fc708aba8aa3R1054

[JalonSolov]

Ah, so opposite of what I thought. Carry on. :-)

[spytheman]

Excellent work.