Skip errors on files with pr URI scheme

[swernli]

Much like git diffs with scheme git, viewing files surfaced from GitHub Pull Requests that come from a branch not currently checked out produces spurious errors since they are not real files on disk and have extra data in their filenames. This uses the same strategy as in #1754 to avoid them by not loading things with the custom pr URI scheme.

[github-actions]

Benchmark for 5d470dc

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Test	Base	PR	%
Array append evaluation	338.9±6.66µs	341.2±10.21µs	+0.68%
Array literal evaluation	170.2±7.07µs	168.8±1.05µs	-0.82%
Array update evaluation	416.5±3.10µs	418.7±7.61µs	+0.53%
Core + Standard library compilation	22.4±0.52ms	22.9±0.71ms	+2.23%
Deutsch-Jozsa evaluation	5.0±0.06ms	4.9±0.08ms	-2.00%
Large file parity evaluation	34.3±0.14ms	34.2±0.11ms	-0.29%
Large input file compilation	13.9±0.22ms	14.1±0.33ms	+1.44%
Large input file compilation (interpreter)	52.6±1.74ms	53.8±2.96ms	+2.28%
Large nested iteration	32.7±0.10ms	32.7±0.27ms	0.00%
Perform Runtime Capabilities Analysis (RCA) on Deutsch-Jozsa sample	1581.9±63.30µs	1576.5±46.28µs	-0.34%
Perform Runtime Capabilities Analysis (RCA) on large file sample	7.9±0.08ms	8.0±0.09ms	+1.27%
Perform Runtime Capabilities Analysis (RCA) on teleport sample	1438.3±40.44µs	1438.5±74.98µs	+0.01%
Perform Runtime Capabilities Analysis (RCA) on the core and std libraries	28.2±0.49ms	27.9±0.75ms	-1.06%
Teleport evaluation	93.7±3.20µs	93.0±3.99µs	-0.75%