Replace full-width prompt header with a variable-width-with-max prompt header

[lihaoyi]

The previous full-width prompt header looks really weird on wide screens, even a full-width laptop screen is enough to make it awkward. This PR replaces it with a variable-width prompt header with some heuristics for shortening the = bars and the title text as necessary.

Covered by existing tests, added some more thorough tests in PromptLoggerUtilTests.scala to exercise the edge cases in the new shortening heuristics

[lihaoyi]

CC @lefou I think we discussed something like this before

[lefou]

I don't need the dashes at all. Also I accidentally thought some time ago that you changed the progress logger to have the seconds before the task name and I really liked it. (Then I realized, I was looking at some evaluator command running anonymous tasks with with extra ticker output.) Nevertheless, If we move the timing between the progress and the task name, we could end up with a condensed consistent output:

Before:

[15624/16847] ================================== __.compile ========================================== 14s
[15133] main.client.compile 3s
[15397] scalanativelib.worker-api.compile 1s
[15404] scalajslib.worker-api.compile 1s
[15612] kotlinlib.worker.test.resolvedIvyDeps
[15613] javascriptlib.test.resolvedIvyDeps
[15614] main.api.test.resolvedIvyDeps
[15615] contrib.versionfile.test.resolvedIvyDeps
[15616] contrib.sonatypecentral.test.resolvedIvyDeps
[15617] contrib.scoverage.worker2.test.resolvedIvyDeps
[15618] contrib.scoverage.api.test.resolvedIvyDeps
[15619] contrib.playlib.worker[3.0].test.resolvedIvyDeps
[15620] contrib.playlib.worker[2.9].test.resolvedIvyDeps
[15621] contrib.playlib.worker[2.8].test.resolvedIvyDeps
[15608] main.eval.test.resolvedIvyDeps
[15611] main.codesig.test.resolvedIvyDeps
[15173] contrib.testng.compile 2s

After (mocked):

[15624/16847]  14s  ==> __.compile
[15133]  3s  main.client.compile
[15397]  1s  scalanativelib.worker-api.compile
[15404]  1s  scalajslib.worker-api.compile
[15612]      kotlinlib.worker.test.resolvedIvyDeps
[15613]      javascriptlib.test.resolvedIvyDeps
[15614]      main.api.test.resolvedIvyDeps
[15615]      contrib.versionfile.test.resolvedIvyDeps
[15616]      contrib.sonatypecentral.test.resolvedIvyDeps
[15617]      contrib.scoverage.worker2.test.resolvedIvyDeps
[15618]      contrib.scoverage.api.test.resolvedIvyDeps
[15619]      contrib.playlib.worker[3.0].test.resolvedIvyDeps
[15620]      contrib.playlib.worker[2.9].test.resolvedIvyDeps
[15621]      contrib.playlib.worker[2.8].test.resolvedIvyDeps
[15608]      main.eval.test.resolvedIvyDeps
[15611]      main.codesig.test.resolvedIvyDeps
[15173]  2s  contrib.testng.compile

[lihaoyi]

For me I need some kind of dashes, otherwise i find the lack of separation makes it visually hard to distinguish log lines from prompt lines.

The timing on the left thing is possible, but one issue is the timing string is variable width, from zero to four digits wide (to give an arbitrary upper limit). So either we would need to reserve a pretty wide gutter, or we would have un-aligned task selectors, or we would have task selectors moving left and right as the longest lived tasks complete. Might be possible to find a compromise but may need more thought

[lefou]

but one issue is the timing string is variable width, from zero to four digits wide

We could use a mm:ss format for the timing.